Applications Manual for the Revised NIOSH Lifting Equation

APPIlCAn~S MANUAL

FOR 1BE REVlSID NIOSH lJFI1NG EQUAn~

Thomas R Waters, Ph.D. Vern Putz-Anderson, Ph.D.

AnnI Garg, Ph.D.

u.s. DEPARIMN'IT OF HFAL1H AND HUMAN SERVIOS PuHic Health Service

Centers for Disease Control and Prevention NationaI Institute for OccupationaI Safety and HeaIth

Division of Biomedical and BehavioraI Science Cincinnati, Ohio 45226

January 1994

DIsa.AIMER

Mention oi the name oi any company or product does not constitute endorsement by the National Institute ior Occupational Safety and Health.

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ii

ACKNOWI.EDGEMENTS

We wish to especially acknowledge the efforts of Arun Garg, Ph.D., Professor of IndustriaI and Systems Engineering, University of Wisconsin-Milwaukee, wIlo served as the chief technical consuItant to NIOSH in the preparation of this docmnent. We a1so wish to acknowledge the technical assistance ofMM Ayoub, Ph.D., Don B. Chaffin, Ph.D., Jerome Congleton, Ph.D., Jeffrey Fernandez, Ph.D., Colin Drury, Ph.D., Stephan Konz, Ph.D., Suzanne Rodgers, Ph.D., and Roger Stephens, Ph.D.

The connnents and reviews provided by other NIOSH personnel are grntefully acknowledged. They include Steven Sauter, Ph.D. and Dan Habes of the Division of Biomedical and Behavirnal Science, and Lawrence 1. Fine, MD., MP.H of the Division of Surveillance, Hazard Evaluations, and Field Studies. We a1so wish to extend OIlI" appreciation to Mr. Ricbard Carlson of the Division of Training and Manpo\\eT Development for the iIIustrations contained in this document. We sincerely appreciate the encouragement and support of Dr. Janet C. Haartz, Director, Division of Biomedical and Behavioral Science.

111

This Manual was deveIoped to provide users ofthe revised NIOSH Iifting equation (1991 version) with methods for accurateIy appIying the Iifting equation to a variety of lifting tasks. AlI neressary terms, definitions, and data requirements for the revised equation are provided in Section 1. Procedures for analyzing singIe-task and IIRJ!ti-task Iifting jooo are described in Section 2. A series of ten lifting tasks is incIuded in Section 3 to illwtrate application of the procedure. For each task, a brief job descrilDon is provided, folICl\\W by a job anaIysis, and a bazanI assessment, incIuding a compIeted ~ Suggestions for redesign of the task are aIso provided.

The rationaIe and supporting criteria for the deveIoplDent of the revised NIOSH lifting equation are described in a journaI articIe, Revised NIOSH Equaionlor the Design md Evduaion 01 Mmud Lifting Tasks, by T. Waters, V. Putz-AncIerson, A Garg, and L Fine, Ergonomics 1993. [See Appendix 1]. The revised equation reflects research finding; pubIished subsequent to the publication of the originai NIOSH equation (1981) and incIudes consideration of additional components of Iifting tasks such as asynnnetricai Iifting and quaIity of hand-container coupIings as weII as a Iarger range of work durations and lifting frequencies tban did the 1981 equation. It must be noted that application of this equation is Iimited to those conditions for -MUch it was designed. It does not, for exarnpIe, address such task factors as one-handed lifting, Iifting extremeIy hot or coId objects, or factors that may increase the risk of a slip or fall and other non-lifting comporellts of job tasks. A complete Iist of work conditions -MUch are 1101 covered by the 1991 equation is presented in Section 1.2 on page 9 ofthis Manual. FinaIIy, it shouId be recognized that alI methods require vaIidation. Appropriate studies for the vaIidation of this equation must be conducted to determine how effective these procedures are in reducing the morbidity associated with manual materials handling.

iv

The equation W<IS designed to assist in the identification of ergonomic solutions for reducing the physicaI stresses associated with manuaI lifting. It is our hope that this ManuaI (1) wiII assist occupationaI safety and health practioners in evaIuating lifting tasks and reducing the incidence of low back injuries in workers, and (2) aIso serve to stimuIate further research and debate on the prevention of low back prin, one of the most costIy occupationaI health problerns facing our nation.

Janet C. Haartz, Ph.D. Director, Division of BiomedicaI and BehavioraI Science

v

TABLE OF <XNIlNfS

ACKNOWLEDGEMENfS . . . . . . . . . . . . . . . . . . . . . . . .. li

liST ofFIGURES . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. viii

liST of TABLES ............................... ix

INìRODUCIlON . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 1

1. TIIE REVISIID IJFDNG EQUATION • • • • • • • • • • • . •• 4 1.1 Definition ofTenrn . . . . . . . . . . . . . . . . . . . . .. 4

1.1.1 Recommended Weight Limit (RWL) ... 4 l.1.2. Lifting Index (Il) . . . . . . . . . . . . . . .. 4 1.1.2. Terminology and Data DefinitiOIl'l .... 5

1.2. Lifting Task LimitatiOIl'l .................. 9 1.3. The Equation and Its FlIllCtion . . . . . . . . . . . .. 12

1.3.1. Horizontal Component . . . . . . . . . . .. 14 1.3.2. Vertica1 Component ............. 17 1.3.3. Distance Component . . . . . . . . . . . .. 18 1.3.4. Asyrmnetry Component . . . . . . . . . .. 19 1.3.5. Frequency Component . . . . . . . . . . .. 22 1.3.6. Coupling Component ............ 28

1.4. The Lifting Index (Il) .................. 33 1.4.1. Using the RWL and li to Guide

&gonomic Design ............. 33 1.4.2. Rationale and LimitatiOll'l for li . . . .. 34 1.4.3. Job-Re1ated Intervention Strategy . . .. 34

2. l'RO(E)l.JlIDì FOR ANAL\'ZING IJFDNG.DJS • • •• 36 2.1. OptiOll'l . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 36

2.1.1. Rationale for Determining Significant Control .... . . . . . . . .. 36

2.1.2. Rationale for Multi-task Analysis Procedure ................... 37

2.2. Collect Data (Step 1) ................... 40 2.3. Single-Task Assessrnent (Step 2) ........... 43 2.4. Multi-Task Procedure ................... 43

VI

2.4.1. Compute the FIRWL for Each Task .. 44 2.4.2. Compute the S1RWL for Each Task .. 44 2.4.3. Compute the F1U for Each Task .... 44 2.4.4. Compute the STIl for Each Task . . .. 45 2.4.5. Compute the CLI for the Job . . . . . .. 45

3. EXAM'lE PROBIDIS ....................... 48 3.1. How to Use the Exarnple Problems ......... 48 3.2. Jobs Perfonned a Few Times Per Shift . . . . . .. 53

3.2.1. Loading Punch Press Stock, Exarnple l . . . . . . . . . . . . . . . . . .. 53

3.2.2. Loading Supply Rolls, Exarnple 2 . . .. 59 3.2.3. Loading Bags Into A Hopper,

Exarnple 3 . . . . . . . . . . . . . . . . . .. 65 3.3. Single Task, Perfonned Repetitively . . . . . . . .. 69

3.3.1. Package Inspection, Exarnple 4 . . . . .. 69 3.3.2. Dish-Washing Machine Unloading,

Exarnple 5 . . . . . . . . . . . . . . . . . .. 73 3.3.3. Product Packaging I, Exarnple 6 . . . .. 79

3.4. Repetitive Multi-Task, Short-Duration . . . . . . .. 84 3.4.1. Depalletizing Operation, Exarnple 7 .. 84 3.4.2. Handling Cans of Liquid, Exarnple 8 . 91

3.5. Repetitive Multi-Task, I..ong-Duration (> 2 brs) . . . . . . . . . . . . . . . . . . . . . . . . .. 99 3.5.1. Product Packaging II, Exarnple 9 .... 99 3.5.2. Warehouse Order Filling,

Exarnple IO . . . . . . . . . . . . . . . . .. 105

GLOSSARY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 113

~CES ................................ 118

APPENDIXI ................................. 121

VII

UST oC FIGURES

Hgure 1 Graphic Representation of Hand Location .. . . . .. 7 Hgure 2 Graphic Representation of

Asymmetry Angle (A) ................ 8 Hgure 3 Single Task Job Analysis Worksheet . . . . . . . . .. 41 Hgure 4 Multi-Task Job Analysis Worksheet .. . . . . . . .. 42 Hgure 5 Loading Ptmch Press Stock, Example 1 . . . . . . .. 54 Hgure 6 Job Analysis Worksheet, Example 1 .. . . . . . . .. 56 Hgure 7 Modified Job Analysis Worksheet, Example 1 .. 58 Hgure 8 Loading Supply Rolls, Example 2 . . . . . . . . . . .. 60 Hgure 9 Job Analysis Worksheet, Example 2 . . . . . . . . .. 61 Hgure lO Modified Job Analysis Worksheet, Example 2 . .. 64 Hgure 11 Loading Bags Into Hopper, Example 3 . . . . . . .. 66 Hgure 12 Job Analysis Worksheet, Example 3 . . . . . . . . .. 68 Hgure 13 Package Inspection, Example 4 ............. 70 Hgure 14 Job Analysis Worksheet, Example 4 . . . . . . . . .. 71 Hgure 15 Dish-Washing Machine Unloading, Example 5 .. 74 Hgure 16 Job Analysis Worksheet, Example 5 . . . . . . . . .. 75 Hgure 17 Modified Job Analysis Worksheet, Example 5 .. 78 Hgure 18 Packaging I, Example 6 .................. 80 Hgure 19 Job Analysis Worksheet, Example 6 ......... 81 Hgure 20 Modified Job Analysis Worksheet, Example 6 . .. 83 Hgure 21 Depalletizing Operation, Example 7 . . . . . . . . .. 85 Hgure 22 Job Analysis Worksheet, Example 7 . . . . . . . . .. 87 Hgure 23 Handling Cans of Iiquid, Example 8 ......... 92 Hgure 24 Job Analysis Worksheet, Example 8 ......... 96 Hgure 25 Product Packaging II, Example 9 ............ 100 Hgure 26 Job Analysis Worksheet, Example 9 . . . . . . . . .. 101 Hgure 27 Warehouse Order Filling, Example lO ........ 106 Hgure 28 Job Analysis Worksheet, Example lO ........ 108

VIli

UST or TABlES

TaNe 1 Horizontal Multiplier .. . . . . . . . . . . . . . . . . . .. 16 TaNe 2 Vertical Multiplier . . . . . . . . . . . . . . . . . . . . . .. 18 TaNe 3 Distance Multiplier ...................... 20 TaNe 4 Asymmetric Multiplier . . . . . . . . . . . . . . . . . . .. 22 TaNe 5 Frequency Multiplier Table (FM) . . . . . . . . . . . .. 26 TaNe 6 Hand-to-Container Coupling Classilication . . . . .. 29 TaNe 7 Coupling Multiplier ... . . . . . . . . . . . . . . . . . .. 31 TaNe 1 Horizontal Multiplier Table (HM) ............ 51 TaNe 2 Vertical Multiplier Table (VM) . . . . . . . . . . . . .. 51 TaNe 3 Distance Multiplier Table (DM) ............. 51 TaNe 4 Asymmetric Multiplier Table (AM) . . . . . . . . . .. 51 TaNe 5 Frequency Multiplier Table (FM) . . . . . . . . . . . .. 51 TaNe 7 Coupling Multiplier Table (CM) ............. 51 TaNe 8 GeneraI DesignlRedesign Suggestions ......... 52

IX

Low back pain (LBP) and injmies attributed to manua! Iifting activities continue as one of the Ieading occupational health and safety issues facing preventive medicine. Despite efforts at controI, inciuding progmm'l directed at both workers and jobs, work-reiated back injmies stilI account for a significant proportion of hmnan suffering and economic cost to this nation. The scope of the probIem was sunnnarized in a report entitled Ba::k l'!iwies, prepared by the Department of Labor's Bmeau of Labor Statistics [DOL(BLS)], Bulletin 2144, published in 1982.

1he DOL's conc1usions are consistent with current workers' compensation data indicating that "injmies to the back are one of the more common and cost1y types of work-related injmies" (National Safety Council, 1990). According to the DOL report, back injmies accounted for nearly 200!o of ali injmies and illnesses in the workplace, and nearly 25% of the annua! workers' compensation payments. A IOOre recent report by the National Safety Council (1990) indicated that overexertion was the most COmIOOn cause of occupational injury, accowrting for 31% of ali injmies. 1he back, IOOreOver, was the body part most frequently injured (22% of 1.7 million injmies) and the most costly to workers' compensation systerns.

Mxe than ten years ago, the National Institute for Occupational Safety and Health (Nl0SH) recognized the growing problem of work-related back injmies and published the Warle Pra::tices Guide far Marud Lifting (NlOSH WPG, 1981). The NlOSH WPG (1981) contained a summary ofthe lifting-related literature before 1981; analytica1 procedures and a lifting eqwtion for ca1culating a recommended \\eight for specified mo-handed, symmetrica1 lifting tasks; and an approach for controlling the ha:zards of low back rryury from manua! lifting. 1he approach to hazard control was coupled to the Action Limit (AL), a resultant tenn that denoted the recommended \\eight derived from the lifting eqwtion

1

In 1985, the National Institute for Occupational Safety and Health (Nl0SH) convened an ad hoc committee of experts \Wc reviewed the current literature 011 lifting, including the NIOSH WPG (1981).1 The literature review was swnmarized in a document entitled Scientific Support Documenlaionfor the Revised 1991 N10SH Lifting Equaion: Technicd Contra:t Reports, Mo/ 8, 1991, which is available from the National Technical InformatiOll Service [NTlS No. PB-91-226-274]. The literature summary contains updated information 011 the physiological, biomechanical, psychophysical, and epidemiological aspects of manua1 lifting. Based on the resu1ts of the literature review, the ad hoc committee recommended criteria for defining the lifting capacity of healthy \\Qdrers. The committee used the criteria to formulate the revised lifting equation The equation was publicIy presented in 1991 by NIOSH staff at a national conference in Ann Arbor, Michigan entitled A Naiond Strdegy for Occupaiond Musculoskeletd Irgury Prevention - Implementction lssues end Resear:h Needs.2

Subsequently, NIOSH staff developed the documentation for the equation and played a prominent mie in recommending methods for interpreting the resu1ts of the lifting equation

The revised lifting equaion reflects new findings end provides methock for evduding arymmetricd lifting t{Eks, end lifts of objects with less tha! optimd couplings between the object end the worlcer's htnds. The revised lifting equaion dso provides guidelines for a more diverse rmge of lifting t{Eks tha! the eaiier equaion (N1OSH Wffi, 1981).

The rationale and criterion for the development of the revised

l The ad hoc 1991 NIOSH Ufting Comnittee meuiler.; inc1uded: MM Ayoob, DonaId B. 0Iaffin, ruin G. Dru!y, Anm Garg. and Stmmne Rodgers. NIOSH Iqxesallalives included Vero PuIz-Andersoo and Thomas R Water.;.

2 For this document, the revised 1991 NIOSH lifting equation will be identified siIqlly as "the revised lifting equatiOiL' The abbreviaIioo WPG (1981) will oontinue lo be used as the reference lo the earlier NIOSH lifting equation, which was documented in a pub1icatioo entitled W",* Pra:tices Guide far Marud Lifting (1981).

2

NIOSH lifting equation are provided in a separate jotnlllll article entitled: Revised N/OSH Equaion for the Design md Evduaion of Marud Lifting Tmks, by Waters, Putz-Anderson, Garg, and Fine, 1993. [Appendix I). We suggest that those practitioners who wish to achieve a better understanding of the data and decisions that were made in fonnulating the revised equation consult the article by Waters et d., 1993. This article provides an explanation of the selection of the biomechanical, physiological, and psychophysical criterion, as well as a description of the derivation of the individuai components of the revised lifting equation. For those individuals, however, who are primarily concemed with the lISe and application of the revised lifting equation, the present docwnent provides a more complete description of the method and limitations for using the revised equation than does the article by Waters et d. 1993. This document: also provides a complete set of examples.

Although the revised lifting equation has not been fully validated, the recommended weight limits derived from the revised equation are consistent with, or lower than, those generally reported in the literature (Waters et d., 1993, Tables 2, 4, and 5). Moreover, the proper application of the revised equation is more likely to protect healthy \IDI'kers for a wider variety of lifting tasks than methods that rely onIy a single task factor or single criterion.

Finally, it should be stressed that the NIOSH lifting equation is only one 1001 in a comprehensive effort to prevent \\UI"k-related low back pain and disability. [Other approaches to prevention are described elsewhere (ASPH'NIOSH, 1986)]. Moreover, lifting is only one of the causes of \\UI"k-related low back pain and disability. Other causes \\hich have been hypothesized or established as risk factors include whole body vibration, static postures, prolonged sitting, and direct trauma to the back. Psychosocial factors, applOpIiate medicai treatment, and job demands (past and present) also may be particularly important in influencing the transition of acute low back pain to chronic disabling pain.

3

1. 1HE REVJSID I.lFIJNG EQUATI<l'J

This section provides the technicd irrformaionfor zmng the revised lifting equaion to evduae a vcriety of two-htnded marud lifting t~ks. Defìnitions, restrictions/limitaions, md dcta requirements for the revised lifting equaion ere dso provided

1.1 Detinition or TelDll

1.1.1 Reconunended Weigbt limit (RWL)

The RWL is the principù product of the revised NIOSH lifting equation. The RWL is defined for a specific set of task conditions as the \\cight of the load that nearly alI healthy \Wrkers could perfonn over a substantial period of time (e.g., up to 8 hours) without an increased risk of developing lifting-related LBP. By hedthy workers, we mean \Wrkers who are free of adverse health conditions that \Wuld increase their risk of muscuIoskeletal injUl)'.

The RWL is defined by the following equation:

RWL= LCX HMXVMX DMXAMX FMX CM

A detailed description of the individuai components of the equation are provided in Section 1.3 on pages 12-13.

1.1.2. lifting Index (li)

The U is a term that provides a relative estimate of the leve! of physica1 stress associated with a particular manuaIlifting task. The estimate of the level of physica1 stress is defined by the relationship of the weight of the load lifted and the recoillilblded

weight limit

4

The U is defined by the following equation:

u = Load Weight __ L_ Recommended Weight Umit RWL

1.1.2. Terminology and Da1a DefinitiOIl'l

The following Iist of brief definitions is useful in applying the revised NIOSH lifting equation. For detailed descriptions of these t=, refer to the individuai sections \\bere each is discussed. ~ for measuring these variables and examples are provided in Sections l and 2.

Inad Weigbt(L)

HorizontaI Location (H)

VeJtical Location (V)

VeJtical Tnwel Dis1ance (D)

AsymmetJy AngIe (A)

Defined as the act of manually grasping an object of definable size and mass with t\m hands, and vertically moving the object without mechanical assistance.

Weight ofthe object to be Iifted, in pounds or kilograrns, including the container.

Distance of the hands away from the mid-point between the ankIes, in inches or centimeters (measure at the origin and destination of lift). See Figme l.

Distance of the hands above the floor, in inches or centimeters (measure at the origin and destination of Iift). See Figme l.

Absolute value of the difference between the vertical heights at the destination and origin of the Iift, in inches or centimeters.

Angular measure of how far the object is displaced from the fiont (mid-sagittal piane) ofthe \\Ul"ker's body at the beginning or ending ofthe Iift, in

5

NeutmI Body Position

degrees (measure at the origin and destination of Iift). See Figure 2. 1he asymmetIy angIe is defined by the location of the load relative to the \\Urker's mid-sagittal pIane, as defined by the neutra! body postm"e, rather than the position of the feet or the extent of body twist.

Describes the position of the body vffien the hands are directly in front ofthe body and there is minimaI twisting at the le~ torso, or shouIders.

lifting Average number of Iifts per minute over a 15 ~ncy (F) minute period.

lifting Three-tiered cJassification of lifting duration Duration specified by the distribution of \\Ul"k-time and

recovery-time (\\UIk pattem). Duration is classified as either short (1 hour), moderate (1-2 hours), or long (2-8 hours), depending on the \\UIk prttern.

CoOlfug CJassification of the quality of the hand-~ect OlMification coupling (e.g., handle, cut-out, or grip). Coupling

quality is classified as good, fair, or poor.

Significant Significant control is defined as a condition Conlrol requiring precision placeJtled of the load at the

destination of the Iift. This is usuaJJy the case vffien (1) the \\UIker has to re-grasp the load near the destination of the Iift, or (2) the \\UIker has to mom::lltmily hold the object at the destination, or (3) the \\UIker has to carefulJy position or guide the load at the destination.

6

Mlo-POINT BE1WEEN INNER ANKLE BONES

TOP VIEW POINT CF

PRo.JECTION

..,....-~/>----HC)RIIZONTAL HORIZONTAL H

f • -LOCATION , •

.-ti LATERA!.

III~NT BE1WEEN INNER ~ BOIIES

YERTICAI. V LOCATION

._ ..... --_::-.. HORIZONTAL

t-H-L HORIZONTAI,. POINT CF PRo.JECTION LOCATION

Hgure 1 Graphic Representation of Hand Location

7

Figure 2 Graphic Representation of Angle of Asymmetry (A)

8

1.2. lifting Tll'Ik limilaDOIl'l

The lifting equation is a tool for assessing the physica1 stress of t\w-handed manua1 lifting tasks. As with any tool, its application is limited to those conditions for which it WcIS designed Specifica1ly, the lifting equation WcIS designed to meet specific lifting-re1ated criteria that encompass biomechanica1, \\Ufk physiology, and psychophysica1 assumptions and data, identified above. To the extent that a given lifting task accmately reflects these underlying conditions and criteria, this lifting equation may be appropriately applied

The following list identifies a set of work conditions in which the application of the lifting equation could either under- or over­estimate the extent of physica1 stress associated with a particular work-re1ated activity. Each of the following task limitations also highlight research topics in need of fìnther research to extend the application of the lifting equation to a greater range of rea! \\Ufld lifting tasks.

L The revised NIOSH lifting equation is based on the assumption that manual handling activities other than lifting are minimal and do not require significant energy expenditure, especially when repetitive lifting tasks are perfonned. Examples of non-lifting tasks include holding, pushing, pulling, carrying, walking, and climbing. 1f such non-lifting activities account for more than about 10"/0 of the total \\Ufker activity, then measures of \\Ufkers' energy expenditures andIor heart rate may be required to assess the metabolic demands of the different tasks. The equation will still apply if there is a small amount of holding and carrying, but carrying should be limited to one or two steps and holding should not exceed a few seconds. For more information on assessing metabolic demand, see Garg et d. (1978) or Eastman Kodak (1986) .

9

2. The revised lifting equation does oot include task factors to account for unpredicted conditiom, such as mexpectedly heavy loads, slipl, or falls. AdditionaI biom:chanical anaIyses may be required to assess the physical stress on joints that occur ftom tramnatic incidents. MJreover, if the enviromnent is unfavorable (e.g., tempeldtures or hmnidity significantly outside the range of 19" to 26"C [660 to 79"F] or 35% to 50%, respectively), independent Iretabolic assessrrents \\OOId be needed to gauge the effects of these variables on heart rate and energy COIlSI.Ullption

3. The revised lifting equation was not designed to assess tasks involving one-handed lifting, lifting \WiIe seated or kneeling, or lifting in a constrained or restricted \\Ufk space.J The equation aIso does not apply to lifting unstable loads. For purposes of applying the equation, an unstable load \\OOId be defined as an object in \Wrich the location of the center of mass varies significantly during the lifting activity, such as some containers of liquid or incompletely filled bags, etc. The equation does oot apply to lifting of wheelbarrows, shoveling, or high-speed lifting.· For such task conditions, independent and task specific biom:chanical, Iretabolic, and psychophysical a.c:sessments may be needed. For information on other assessment methods, refer to Eastman Kodak (1986), Ayoub and MitaI (1989), 0Iaffin and Andersson (1991), or Snook and Ciriello (1991).

4. The revised lifting equation assmnes that the worker/floor surface coupling provides at least a 0.4 (preferably 0.5) coefficient of static friction between the shoe sole and the working smface. An adequate \\Ufker/floor smface coupling is necessary \\hen. lifting to provide a finn footing and to control accidents and

, The resean:It SIaIf of the Bureau of Mines bave pubIished lIUIIICfOUS studies 00 lifting 1Mille lmeeling and in n:sIricted worIcspac<s (Sa: GaIIagha .t d., 1988; GaIIagha and Unger, 1990; and, GaIIagha, 1991).

• AIthough lifting speed is diffiru1t lo judge, a high speed Iift wooId be equivalertt lo a speed of about 30 incbesIsecood. f<Jr ~ purposes, a Iift &cm the floce lo a 1ab1o-1Dp 1bat is aJrIlIIeted in Iess !han about I secood wooId be oonsidered high speed.

lO

~uries resulting from foot slippage. A 0.4 to 0.5 coefficient of static friction is comparable to the friction found between a smooth, dry tloor and the sole of a clean, dry leather WJrk shoe (nonslip type). Independent biomechanicaI nxxIeling may be used to account for variations in the coefficient of friction.

5. The revised lifting equation assumes that lifting and lo\WIÙlg tasks bave the same level of risk for low back ~uries (i.e. that lifting a box from the tloor to a table is as ha7Judous as lo\WIÙlg the same box from a table to the tloor). This asswnption may not be true if the \\UIker actually drops the box rather than lowering it alI the way to the destination. Independent metabolic, biomechanicaI, or psychophysicaI assessments may be needed to assess \\UIker capacity for various lowering conditions. (See references provided above.)

In swnmary, the Revised NIOSH Lifting Equation does not apply if any of the following occur:

• Liftingllo\WIÙlg with one band

• Liftingllowering for over 8 hours

• Liftingllowering while seated or kneeling

• LiftingIlo\WIÙlg in a restricted \\UI"k space

• LiftingIlowering unstable objects

• LiftingIlowering while carrying, pusbing or pulling

• LiftingIlo\WIÙlg with wbeelbarrows or shovels

• Liftingllowering with high speed motion (faster than about 30 inchesIsecoOO)

• Liftingllowering with unreasonable footltloor coupling « 0.4 coefficient of friction between the sole and the tloor)

11

• Liftinw'lowering in an unfavorable enviromnent (Le., temperatme significantly OIJtside 66-79" F (19-26° C) range; relative hmnidity outside 35-50"/0 range)

For those 1ifting tasks in \\bich the applicatiOll of the revised 1ifting equatiOll is not appropriate, a more comprehensive ergonomic eva1uatiOll may be needed to quantify the extent of other physica1 stressors, such as prolOllged or fi:equent non-neutral back postures or seated postures, cyclic loading (\\baIe body vibrntiOll), or unfavorable environmental factors (e.g., extreme heat, cold, hmnidity, etc.).

Any of the above factors, alone or in combinatiOll with manual lifting, may exacerOOte or inìtiate the onset of low back pain

1.3. 1be &ption and l1s Function

The revised lifting equatiOll for ca1cu1ating the Recommended Weight Limit (RWL) is based on a multiplicative IIlOdel that provides a weighting for each of six task variables. The weighting:; are expressed as coefficients that serve to decrease the load constant, \\bich represents the maximwn I"eCOIIIIreIlde load weight to be lifted llIIder ideai conditions. The RWL is defined by the following equation:

RWL = LC X HM X VM X DM X AM X FM X CM

12

Where:

METRlC uso CUSTOMARY

Load LC 23 kg 51 Ib Constant

Horizontal HM (25/H) (lO/H) Multiplier

Vertical VM 1-(.003 IV-75 I) 1-(.0075Iv-30 I> Multiplier

Distance DM .82 + (4.5/0) .82 + (1.8/0) Multiplier

Asymmetric AM 1-(.0032A) 1-(.0032A) Multiplier

Frequency FM From Table 5 From Table 5 Multiplier

Coupling CM From Table 7 From Table 7 Multiplier

The term task variobles refers to the measurable task descriptors (i.e., H, V, D, A, F, and C); \Wereas, the term mUtipliers refers to the reduction coefficients in the equation (i.e., HM, VM, DM, AM, FM, andCM).

Each multiplier shouJd be computed from the appropriate fomtula, but in some cases it will be necessary to use Jinear interpolation to determine the value of a multiplier, especially when the value of a variable is not directly avallable fium a table. For exarnple, when the measured frequency is not a whole number, the appropriate multiplier nrust be interpolated between the frequency values in the table for the t\\Q values that are closest to the actua1 frequency.

A brief discussion 01 the task vCTiciJles, the restrictions, aul the associcted multiplier lor ea:h component 01 the model is presented in the lollowing sections.

13

1.3.1. Horizonlal CoJl1lOnent

1.3.1.L Definition and l'tbW\!meot

Horizonlall.ocalion (H) is measured from the mid-point of the 1ine joining the inner ankIe bones to a point projected on the fIoor directly beIow the mid-point of the band grasps (i.e., Ioad center), as defined by the large middle knuclde of the band (Figure 1). Typically, the \\Urker's feet are not aligned with the mid-sagittal pIane, as shown in Figure 1, but may be rotated inward or outward. If this is the case, then the mid-sagittal pIane is defined by the \\Ul"ker's neutra! body posture as defined above.

If significant controi is required at the destination (i.e., precision piacem::nt), then H should be measured at both the origin and destination ofthe lift

Horizonlall.ocalion (H) sbould be _ured In those situations where the H value can not be measured, then H may be approximated from the following equations:

Metric [Ali distances in anI

H = 20 + W/2 forV ~25 cm

H = 25 +W/2 forV < 25 cm

li5. Customary [AlI distances in inchesl

H - 8 +W/2 for V ~ 1 O inches

H - lO +W/2 for V < lO inches

Where: W is the width of the container in the sagittal piane and V is the vertica1 Iocation of the hands from the fIoor.

1.3.1.2. Horizonlal Restrictions

Ifthe horizontal distance is less than lO inches (25 cm), then H is set to IO inches (25 cm). A1though objects can be carried or heId c10ser than lO inches from the ankIes, most objects that are c10ser than this cannot be Iifted without encowrteri.ng interference from

14

the abdomen or hyperextending the shoulders. While 25 inches (63 cm) was chosen as the maximum value for H, it is probably too large for sborter \\UI'kers, particu1arly \\<ben lifting asynnnetricaJJy. Furthennore, objects at a distance of more than 25 inches from the ankIes nonnaJly cannot be lifted verticaJJy without some loss of balance.

1.3.1.3. lilrizonlal Multipier

The Horizontal MuItiplier (HM) is 101H, for H measured in inches, and HM is 25/H, for H measured in centimeters. Jf H is Iess than or t!l/ld lo lO inches (25 cm), then the nruJtiplier is 1.0. HM decreases with an increase in H value. The multiplier for H is reduced to 0.4 when H is 25 inches (63 cm). 1f H is greater than 25 inches, then HM = O. The HM value can be computed directJy or detennined from Table 1.

15

Table 1 IlIrizonlal Mul1ipier

H HM H HM in cm

~o 1.00 !S25 1.00 11 .91 28 .89 12 .83 30 .83 13 .77 32 .78 14 .71 34 .74 15 .67 36 .69 16 .63 38 .66 17 .59 40 .63 18 .56 42 .60 19 .53 44 .57 20 .50 46 .54 21 .48 48 .52 22 .46 50 .50 23 .44 52 .48 24 .42 54 .46 25 .40 56 .45

>25 .00 58 .43 60 .42 63 .40

>63 .00

16

1.3~ Vel1ical Coqxlllent

1.3.2.1. Definition and Mea<iurement

Vel1ical l.ocation (V) is defined as the vertical height of the bands above the floor. V is measured vertically from the floor to the mid-point between the band grasps, as defined by the large middle knuckle. The coordinate system is illustrated in Figw-e l (page 7).

1.3.2.2. Vel1ical RestrictiOIl'i

The vertical location (V) is limited by the floor surface and the upper limit ofvertical reach for lifting (i.e.,70 inches or 175 cm). The vertical location should be measured at the origin and the destination of the lifi to detennine the travel distance (O).

1.3.2.3. Vel1ical Multipier

To detennine the Vertical Multiplier (VM), the absolute value or deviation of V from an optimum height of 30 inches (75 cm) is calcu1ated. A height of 30 inches above floor level is considered "knuckle height" for a worker of average height ~66 ~hes or 165 cm). The Vertical Multiplier (VM) is (~·{.007~ IV-30 I)) for V measured in inches, and VM is (l-(.003 IV-75 I)), for V measured in centimeters.

When V is at 30 inches (75 cm), the vertical multiplier (VM) is 1.0. The value of VM decreases linearly with an increase or decrease in height from this position At floor leve!, VM is 0.78, and at 70 inches (175 cm) height VM is 0.7. IfV is greater than 70 inches, then VM = O. The VM value can be computed directly or determined from Table 2.

17

v in O 5 lO 15 20 25 30 35 40 45 50 55 60 65 70

>70

ThbIe 2 Vertical Multipier

VM V cm

.78 O

.81 lO

.85 20

.89 30

.93 40

.96 50 1.00 60 .96 70 .93 80 .89 90 .85 100 .81 110 .78 120 .74 130 .70 140 .00 150

160 170 175

>175

1.3.3. Dis1lmce Ch~nent

1.3.3.1. Definition and ~urement

VM

.78

.81

.84

.87

.90

.93

.96

.99

.99

.96

.93

.90

.87

.84

.81

.78

.75

.72

.70

.00

The Vel1ical 1ìavel Dis1lmce variable (O) is defined as the vertica1 travel distance of the hands bet\\een the origin and destination of the lift. For lifting, D can be computed by subtracting the vertica1 location (V) at the origin of the lift from the conesponding V at

18

the destination of the lift (Le., D is equa! to V at the destination minus V at the origin). For a lom:ring task, D is equa! to V at the origin minus V at the destination.

1.3.3.2 Dis1ance Restrictiom

The variable (D) is assumed to be at least lO inches (25 cm), and no greater than 70 inches [175 cm]. 1f the vertica1 travel distance is less than IO inches (25 cm), then D should be set to the minimum distance of lO inches (25 cm).

1.3.3.3 Dis1ance Multi~er

The Distance Multiplier (DM) is (.82 + (1.8ID» for D measured in inches, and DM is (.82 + (4.5/0» for D measured in centimeters. For D less than lO inches (25 cm) D is assumed to be lO inches (25 cm), and DM is 1.0. The Distance Multiplier, therefore, decreases graduaIly with an increase in travel distance. The DM is 1.0 when D is set at lO inches, (25 cm); DM is 0.85 when D = 70 inches (175 cm). Thus, DM ranges from 1.0 to 0.85 as the D varies from O inches (O cm) to 70 inches (175 cm). The DM value can be computed directly or determined fiom Table 3.

1.3.4. AsymmetJy (hqxlllent

1.3.4.1. Definition and Measurement

Asymmetry refers to a lift that begins or ends outside the mid­sagittal piane as shown in Figure 2 on page 8. In genernl, asymmetric lifting should be avoided. 1f asymmetric lifting cannot be avoided, however, the recommended weight limits are significantly less than those limits used for symmetrica1 lifting.s

, Il may oot a1ways be clear if asymnetIy is an intrimic element of!be task or just a per.;onaI cbaracteristic of!be worI<fts lifting style. Regardless of!be rea<;()1l for !be asymnetIy. any observed asynmetric lifting shoold be coosidered an intrimic element of!be job design and shoold be coosidered in !be asstSSi,,,,tt and suIlsequent redesign. Mlreovc-, !be design of!be task shoold oot rely 00 woda:r ~iance, but raIher !be design shoold discourage or eliminate !be need for asynmetric lifting.

19

Table 3 Dis1ance Mul1iplier

D DM D DM in cm

:90 1.00 ~5 1.00 15 .94 40 .93 20 .91 55 .90

25 .89 70 .88

30 .88 85 .87

35 .87 100 .87 40 .87 115 .86 45 .86 130 .86

50 .86 145 .85 55 .85 160 .85

60 .85 175 .85 70 .85 >175 .00

>70 .00

An asymrnetric lift may be required under the following task or \\\Jfkplace conditions:

1. The origin and destination of the lift are oriented at an angIe to each another.

2. The lifting motion is across the body, such as occurs in swinging bags or boxes from one Iocation to another.

3. The lifting is done to maintain body balance in obstructed \\\Jfkplaces, on rough terrain, or on littered floors.

4. Productivity standards require redllced time per lift.

20

The asymmetric angle (A), which is depicted graphicaIly in Figure 2, is operationa11y defmed as the angle between the asymmetry line and the mid-sagittalline. The asymmetTy fine is defined as the horizontal line that joins the mid-point between the inner ankle bones and the point projected on the floor directly below the mid­point of the band grasps, as defined by the large middle knuckle.

The sagittd fine is defined as the line passing through tbe mid­point between the inner ankle bones and lying in the mid-sagittal piane, as defined by the neutra! body position (i.e., hands directly in front of the body, witb no twisting at the legs, torso, or shou1ders). Note: The asymrnet:ry angle is not defined by foot position or the angIe of torso twist, but by the location of the load relative to the worker's mid-sagittal piane.

In many cases of asynunetric lifting, the worker will pivot or use a step twn to complete the Iift. Since this may vary significant1y between workers and between lifls, we bave assumed that no pivoting or stepping occurs. Although this assumption may overestimate the reduction in acceptable load weight, it will provide the greatest protection for the worker.

The asymrnet:ry angle (A) rnust a1ways be measured at the origin of the 1ift. If significant control is required at the destination, however, then angle A shou1d be measured at both the origin and the destination of the lift.

1.3.4.2. AsymmetJy Restrictiom

The angle A is limited to the range from 0° to 135°. If A> 135°, then AM is set equa! to zero, which results in a RWL of zero, or no load.

1.3.4.3. Asymmetric MultiPier

The Asynunetric Multiplier (AM) is 1-(.OO32A). The AM has a maximum value of 1.0 \\ben the load is lifted directly in front of

21

the body. The AM decreases linearly as the angle of asymmetry (A) increases. The range is from a value of 0.57 at 135° of asymmetry to a value of 1.0 at 00 of asymmetry (i.e., symmetric lift).

1f A is greater than 135°, then AM = O, and the load is zero. The AM value can be computed directly or detennined from Table 4.

Table 4 Asymmetric Multiplier

A AM deg

O 1.00

15 .95

30 .90

45 .86

60 .81

75 .76

90 .71

105 .66

120 .62

135 .57

>135 .00

1.3.5. Frequency Co~nent

1.3.5.1 Definition and Measurement

The frequency multiplier is defined by (a) the number of lifts per minute (frequency), (b) the amount oftime engaged in the lifting activity (duration), and (c) the vertical height ofthe lift from the floor. Lifting frequency (F) refers to the average number of lifts made per minute, as measured over a 15-minute period. Because

22

of the potential variation in work patterns, anaIysts may bave difficulty obtaining an accurate or representative 15-minute work sample for computing the lifting frequency (F). li' significant variation exists in the frequency of lifting over the COW"Se of the day, anaIysts should employ standard work sampling tecbniques to obtain a representative work sample for determining the number of lifts per minute. For those jobs where the frequency varies from session to session, each session should be anaIyzed separately, but the overali work pattem lllI.L5t still be considered. For more information, most standard industria1 engineering or ergonomics texts provide guiclance for establishing a representative job sampling strategy (e. g., Eastman Kodak Company, 1986).

1.3.5.2 lifting Dumtion

Lifting duration is c1assified into three categories--short -duration, rnoderate-duraton and long-duration. These categories are based on the pattem of continuous work-time and 1I!coveJY-time (i.e., light work) periods. A continuous work-time period is defined as a period of unintemJpted work. Recovery-time is defined as the duration of light work activity following a period of continuous lifting. Examples of light work include activities such as sitting at a desk or table, monitoring operations, light assembly work, etc.

1. Sbort-duration defines lifting tasks tbat bave a work duration of one ho" or less, followed by a recovery time equa! to 1.2 times the work time [i.e., at least a 1.2 recovery-time to work-time ratio (RTIWI)].

For example, to be classified as short-duration, a 45-minute lifting job lllI.L5t be followed by at least a 54-minute recovery period prior to initiating a subsequent lifting session. li' the required recovery time is not met for a job of one hour or less, and a subsequent lifting session is required, then the tota! lifting time lllI.L5t be combined to correctly determine the duration category. Mlreover, if the recovery period does not rneet the time requirement, it is disregarded for purposes of determining the applOptiate duration category.

23

As another example, assume a \\QI"ker lifts continuously for 30 minutes, then perfOIlllS a light \\Ul"k task for lO minutes, and then lifts for an additional 45-minute period. In this case, the recovery time ~ lifting sessions (lO minutes) is less than 1.2 times the initial 30-minute \\Ul"k time (36 minutes). Thus, the two work times (30 minutes and 45 minutes) must be added together to determine the dmation. Since the total \\Ul"k time (75 minutes) exceeds l hour, the job is classified as moderate-duration. On the other band, if the recovery period ~ lifting sessions was increased to 36 minutes, then the short-duration category \\Quld apply, \\hlch \\Quld result in a larger FM value.

2. Modemfe.dmdion defines lifting tasks that bave a dmation of more than one ho", hli noi more than two holl'S, followed by a recovery period of at least 0.3 times the \\Qrk time [i.e., at least a 0.3 recovery-time to \\QI"k-time ratio (RTIWI)].

For example, if a \\Urker continuously lifts for 2 hours, then a recovery period of at least 36 minutes \\Quld be required before initiating a subsequent lifting session. Ifthe recovery time requirem:nt is not met, and a subsequent lifting session is required, then the totaI \\QI"k time must be added together. If the total \\QI"k time exceeds 2 hours, then the job must be classified as a long­dmation lifting task.

3. l.ong-Wrntion defines lifting tasks that bave a dmation of between two and eight holl'S, with standard industriai rest allowances (e.g., moming, lunch, and aftemoon rest breaks).

Note: No weigbt lliUts are puvided Cor more tban eigbt boms oC work

The difference in the required RTIWf ratio for the short-duration category (less than l hour), \\hlch is 1.2, and the moderate­dmation category (1-2 hours), \\hlch is .3, is due to the difference in the magnitudes of the frequency multiplier values associated with each of the dmation categories. Since the moderate-duration category results in larger reductions in the RWL than the short-

24

duration category, there is less need for a recovery period between sessions than for the short duration category. In other \\UlÙs, the short duration category would result in higher weight lirnits than the moderate duration category, so larger recovery periods would be needed.

1.3.5.3. FleIpncy Reslrictions

Lifting frequency (F) for repetitive lifting may range from 0.2 lifts/min to a maximmn frequency that is dependent on the vertica1 location of the object (V) and the duration of lifting (fable 5). Lifting above the maximmn frequency results in a RWL of 0.0. (Except for the special case of discontinuous lifting discussed above, \\bere the maximwn frequency is 15 lifislminute.)

1.3.5.4. Frequency Multipier

The FM value depends upon the average nwnber of lifts/min (F), the vertica1 location (V) of the hands at the origin, and the duration of continuous lifting. For lifting tasks with a frequency less than .2 lifts per minute, set the frequency equal to .2 lifts/minute. For infrequent lifting (i.e., F < .1 liftIrninute), however, the recovery period will usua1ly be sufficient to use the l-hour duration category. The FM value is determined from Table 5.

25

Table 5 Frequency Multipier Table (FM)

Frequerk.y Work OUraoon Ufts/min "1 Hour >1 but,,2 Hours >2 but,; 8 Hours

(fH V < 30t V~30 V < 30 V230 V < 30 V~30

,;Q. 2 1.00 1.00 .95 .95 .85 .85 0.5 .97 .97 .92 .92 .81 .81

I .94 .94 .88 .88 .75 .75 2 .91 .91 .84 .84 .65 .65 3 .88 .88 .79 .79 .55 .55 4 .84 .84 .72 .72 .45 .45 5 .80 .80 .60 .60 .35 .35 6 .75 .75 .50 .50 .27 .27 7 .70 .70 .42 .42 .22 .22 8 .60 .60 .35 .35 .18 .18 9 .52 .52 .30 .30 .00 .15 IO .45 .45 .26 .26 .00 .13 11 .41 .41 .00 .23 .00 .00 12 .37 .37 .00 .21 .00 .00 13 .00 .34 .00 .00 .00 .00 14 .00 .31 .00 .00 .00 .00 15 .00 .28 .00 .00 .00 .00

>15 .00 .00 .00 .00 .00 .00

tValues ofV are in inches. :For lifting less frequentIy than ooce per 5 minutes, sa F = 2 lifts/minute.

26

1.3.5.5. Special Frequency At:Ijtfitment Procemre

A speciti ptrJCl!l!we has been developed for determining the appropriate lifting frequency (F) for certain repetitive lifting tasks in which worlrers do not Iift continuously during the 15 minute sampling period. This occurs \\ben the \\Ul"k pattem is such that the \\Ul"ker: lifts repetitively for a short time and then perform; light \\Ul"k for a short time before starting another cycle. As long as the actual lifting frequency does not exceed 15 lifts per minute, the lifting frequency (F) may be detennined for tasks such as this as follows:

l. Compute the total number of lifts perfonned for the 15 minute period (i.e., lift rate times \\Ul"k time).

2. Divide the total number of lifts by 15.

3. Use the resulting value as the frequency (F) to determine the frequency multiplier (FM) from Table 5.

For example, if the \\Ul"k pattem for a job consists of a series of cyclic sessions requiring 8 minutes of lifting followed by 7 minutes of light work, and the lifting rate during the \\Ul"k sessions is IO lifts per minute, then the frequency rate (F) that is used to determine the frequency multiplier for this job is equa! to (lO x 8YI5 or 5.33 liftslminute. If the worker Iifted continuously for more than 15 minutes, however, then the actuallifting frequency (lO lifts per minute) would be used.

When using this special procedure, the duration category is based on the magnitude of the recovery periods between work sessions, not within work sessions. In other words, if the work pattem is intermittent and the special procedure applies, then the intermittent recovery periods that occur during the I5-minute sampling period are not considered as recovery periods for purposes of determining the duration category. For example, if the work pattem for a manual lifting job WcIS composed of repetitive cycles consisting of I minute of continuous lifting at a rate of lO liftslminute, followed

27

by 2 minutes of recovery, the correct procedure \\OOld be to adjust the frequency according to the special procedure [i.e., F = (lO liftsIminute x 5 minutesy15 minutes = 50115 = 3.4 liftsIminute.] The 2-minute recovery periods \Wuld not count towards the WfIRT ratio, however, and additional recovery periods \Wuld bave to be provided as described above.

1.3.6. Couping CoqlOnent

1.3.6.1. Definition & Meaoiurement

The nattn"e of the band-to-object coupling or gripping method can affect not only the maxinnun force a worker can or must exert on the object, but aJso the verticaI location of the hands dwing the Iift. A good coupling wilI reduce the maxinnun grasp fon:es required and increase the acceptable weight for lifting, while a poor coupling wilI generaIIy require higher maxinnun grasp fon:es and decrease the acceptable \\eight for lifting.

The effectiveness of the coupling is not static, but may vary with the distance of the object from the ground, so that a good coupling could become a poor coupling during a single Iift. The entire range of the Iift shouId be COIl'lidered \\ben cIassifying band-to­object coupling;, with cIassification based on overaII effectiveness. The ana1yst must cIassify the coupling as good, fair, or poor. The three categories are defined in Table 6. If there is any doubt about cIassifying a particuIar coupling design, the more stressful cIassification shouId be selected.

28

Table 6 Hmd-to-Con1ainer Couping ~ification

GOOD FAIR l'OOR

1. For containers 1. For containers of 1. Containers of of optimal design, optimal design, a less than optimal such as some boxes, ''Fair'' hand-to- design or loose crntes, etc., a object coupJing parts or irreguIar "Good" hand-to- \muld be defined as o~ects that are object coupling handles or hand- bulky, hard to \muld be defined as hold cut-outs of less handle, or have handles or hand- than optimal design sharp edges [see hold cut -outs of [see notes l to 4 note 5 below]. optimal design [see below]. notes 1 to 3 below].

2. For loose parts 2. For containers of 2. Lifting non-rigid or irreguIar o~ects, optimal design with bag; (i.e., bag; that \\bich are not no handles or hand- sag in the middJe). usually hold cut-outs or for containerized, such loose parts or as castings, stock, irreguIar objects, a and supply "Fair" hand-to-materials, a "Good" object coupling is hand-to-object defined as a grip in coupling \muld be \\bich the hand can defined as a be flexed about 90 comfortable grip in degrees [see note 4 \\bich the hand can below]. be easily wrapped around the object [see note 6 below].

29

l. An optirnal handle design has .75 - 1.5 inches (1.9 to 3.8 cm) diameter, ~ 4.5 inches (11.5 cm) Iength, 2 inches (5 cm) clearance, cylindrica1 shape, and a smooth, non-slip swface.

2. An optimal hand-hoId cut-out has the following approximate characteristics: ~ 1.5 inch (3.8 cm ) height, 4.5 inch (11.5 cm) length, semi-oval shape, ~ 2 inch (5 cm) clearance, smooth non­slip swface, and ~ 0.25 inches (0.60 cm) container thickness (e.g., double thickness cardboord).

3. An optimal container design has ~ 16 inches (40 cm) frontaI length, ~ 12 inches (30 cm ) height, and a smooth non-slip surface.

4. A worker should be CllJXlble of clamping the fingers at nearly 90" under the container, such as required \\ben lifting a cardboard box from the floor.

5. A container is considered less than optimal if it has a frontaI length> 16 inches (40 cm), height > 12 inches (30 cm), rough or slippery swfaces, sharp edges, asymmetric center of mass, IlI1SIable contents, or requires the use of gloves. A loose object is considered bulky if the load cannot easily be balanced between the hand-grasps.

6. A worker should be abie to comfortabIy wrap the hand around the ~ect without causing excessive wrist deviations or awkward postures, and the grip should not require excessive force.

30

1.3.6.2. Couping Multipier

Based on the coupling c1assifkation and vertical location of the lift, the Coupling Multiplier (CM) is detennined from Table 7.

Coupling Type

Good

Fair

Poor

TaHe7 Couping Multipier

Coupling Multiplier

V< 30 inches V ~ 30 inches ( 75 cm) (75 cm)

1.00 1.00

0.95 1.00

0.90 0.90

31

1he following decision tree may be helpful in cIassifying the band­to-o~ect coupling.

I Container

I

Oplimal Container?

VES I

Optimal BandIe.?

VES

Decision Tree for Coupling Quality

Object Lifted

I 1.00 .. Object

I NO VES BuIky

Object?

NO

I POOR I Optlmal

Gri • p.

NO NO NO VES

Finge .. '-- Flexed f------

90 degrees?

§J FAIR

I GOOD l I

32

I

1.4. The lifting Jndex (Il)

As defined earlier, the Lifting Index (Il) provides a relative estimate of the physica1 stress associated with a manual lifting job.

u = load Weight Recommended Weight Umit

L =

RWL

Where Load Weigbt (L) = weight of the object lifted (lbs or kg).

1.4.1. Using tbe RWL and U Co Guide Fìgonomic Design

The recommended weight limit (RWL) and lifting index (Il) can be used to guide ergonomic design in severa1 ways:

(1) The individuai multipliers can be used to identify specific job­related problems. The relative magnitude of each multiplier indicates the relative contribution of each task factor (e.g., horizontal, vertica1, frequency, etc.)

(2) The RWL can be used to guide the redesign of existing manual lifting jobs or to design new manuallifting jobs. For example, if the task variables are fixed, then the maximum weight of the load could be selected so as not to exceed the RWL; if the weight is fixed, then the task variables could be optimized so as not to exceed the RWL.

(3) The il can be used to estimate the relative magnitude of physica1 stress for a task or job. The greater the U the smaller the fraction of workers capable of safe1y sustaining the level of activity. Thus, two or more job designs could be compared.

(4) The il can be used to prioritize ergonomic redesign. For example, a series of suspected hazardous jobs could be rank ordered according to the il and a control strategy could be deve10ped according to the rank ordering (i.e., jobs with lifting

33

indices above LO or higher \Wuld benefit the most fiom redesign).

1.4.2 Rafionale md limitatiom for U

The NIOSH RI:coInm:Id:d Weight Limit (RWL) equation and Lifting Index (Il) are based on the conrept that the risk of lifting­reIated low back pain increases as the demands of the lifting task ÌllCrease. In other VIUl"ds, as the magnitude of the Il increases, (1) the level of the risk for a given \\Orker \Wuld be increased, and (2) a greater percentage of the workforce is likely to be at risk for developing lifting-reIated low back pain The shape of the risk fimction, ~er, is not known. Without additiona1 data showing the relationship between low back pain and the Il, it is impossible to predict the magnitude of the risk for a given individuai or the exact perc:ent of the \\Ul"k popuIation \\ho would be at an elevated risk for low back pain

To gain a better understanding of the rationale for the development of the RWL and Il, consult the paper entitled Revised NIOSH Equaianlor the Design ad Evduaion 01 Marud Lifting Tasks by Waters, Putz-Anderson, Garg, and Fine (1993) (Appendix I). This article provides a discussion of the criteria underlying the lifting equation and of the individuai muItipliers. This article also identifies both the asswnptiom and uncertainties in the scientific studies that associate manual lifting and low back ~uries.

1.4.3 • .bb-ReIated InrelVenlion Strnregy

The lifting index may be used to identify potentially hazardous lifting jobs or to compare the relative severity of t\\Q jobs for the purpose of evaluating and redesigning them From the NIOSH perspa:tive, it is likely that lifting tasks with a Il > LO pose an increased risk for lifting-related low back pain for some fraction of the workforce (Waters et d., 1993). Hence, the ~ shouId be to design ali lifting jobs to achieve a Il of LO or lesso

34

Some experts believe, however, that \\Ul"ker selection criteria may be used to identify workers \\ho can perfonn potentially stressful lifting tasks (i.e., lifting tasks that \\Ould exceed a il of 1.0) without significantly increasing their risk of \\Ol"k-related injmy (Chaffin and Anderson, 1984; Ayoub and Mital, 1989). Those selection criteria, however, must be based on research studies, empirical observations, or theoretical considerations that include job-related strength testing an:I/or aerobic capacity testing. Nonetheless, these experts agree that nearly ali workers will be at an increased risk of a \\Ol"k-related ÌIlimy when performing highly stressfullifting tasks (i.e., lifting tasks that \\Ould exceed a il of 3.0). Also, irfomd or ndllri selection of workers may occur in many jobs that require repetitive lifting tasks. According to some experts, this may resuit in a II!lÌque \\Orkforce that may be able to \\Ork above a lifting index of LO, at least in theory, without substantially increasing their risk of low back injuries above the baseline rate of injmy.

35

2. PROCEDURES FOR ANALY7JNG llFI1NG.xJBS

This section describes the procedures tha shauld be lollowed to correctly assess the physicd demmds 01 a mmud liftingjob.

2.1. OptiOIl'l

Prior to the assessment, the ana1yst must determine (I) ifthe job should be ana1yzed as a single-task or multi-task manuallifting job, and (2) if significant control is required at the destination of the lift.

A single-task manuallifting job is defined as a lifting job in which the task variables do not significantly vary from task to task, or oniy ODe task is of interest (e.g., worst case ana1ysis). This may be the case if the effects of the other tasks on strength, localim! muscle fatigue, or whole-body fatigue do not differ significantly from the worst case task.

On the other band, multi-task manual lifting jobs, which are defined as jobs in which there are significant differences in task variables between tasks, are more difficult to ana1yze boo!!lse each task must be ana1yzed separately. Therefore, a specialized JrOCedure is used to ana1yze multi-task manuallifting jobs.

2.1.1. Rationale for DetenniningSignificant COntrol

When significant control of an object is required at the destination of a lift, the v.urker must apply a significant upward force to decelerate the object. Depending upon the velocity of the lift, this deceleration force may be as great as the force required to lift the object at the origin Therefore, to insure that the appropriate RWL is computed for a lift that requires significant control at the destination, the RWL is calculated at both the origin and the destination of the lift, and the ltmer of the t\W values is used to assess the overa11lift. The latter JrOCedure is required if (I) the v.urker has to re-grasp the load near the destination of the

36

lift, (2) the worker has to momentarily hold the object at the destination, or (3) the worker has to position or guide the load at the destination The pmpose of calculating the RWL at both the origin and destination of the lifi is to identify the most stressful location of the lift.

2.1.2. Rationale for MuIti-ta'lk Analysis Procedure

The initial recommendation for anal)Zing the physical dernands of multi-task manualliftingjobs was included in the NIOSH WPG (1981). The procedW'e was designed to determine the collective effects of alI the tasks. The procedW'e included: (l) detennining a frequency-weighted average for each task variable; (2) deterrnining each of the fOlli" multipliers, the AL and the MPL, using the frequency-weighted average variables; and, (3) comparing the frequency-weighted average weight with the AL and MPL. The averaging approacb, however, can rnask the effects of hazardous task variables, resu1ting in an underestimation of the lifting hazard (Waters, 1991). For example, consider a rnulti-taskjob consisting of two separate tasks, each with a frequency of l lift/rninute and vertical heights (V) of O and 60 inches. Although both tasks considered individually would have large penalties for the vertical height factor, \\ben cornbined in this rnanner the frequency­weighted (average) V is 30 inches, which cancels the penalty for vertical height, resu1ting in no reduction in the recornrnended weight lirnit. Because of the potential inaccuracies that can occur \\ben task variables are averaged for multi-task assessrnents, a new rnulti-task rnethod was developed. The rnethod is described onpage 43.

The new rnethod is based on the following assumptions:

1. That perforrning multiple lifting tasks would increase the physical or rnetabolic load, and that this increased load should be reflected in a reduced recornrnended weight lirnit and increased Lifting Index.

37

2. That an increase in the Lifting Index depends upon the characteristics of the additional lifting task.

3. That the increase in the Lifting Index due to the addition of one or more tasks is independent of the Lifting Index of any of the preceding tasks (i.e., Lifting Indices from tasks already perfrnmed).

Although the procedure does not consider the potential interaction between individuai lifting tasks, \\e believe this effect is minimal.

The new rnethod is based on the concept that the Composite Lifting Index (CLI), which represents the collective demands of the job, is equa! to the sum of the largest Single Task Lifting Index (S111) and the incrementaI increases in the CLI as each subsequent task is added. The incrementai increase in the CLI for a specific task is defined as the difference between the Lifting Index for that task at the cumulative frequency and the Lifting Index for that task at its actua1 frequency. For exarnple, consider two identica1 tasks (A and B), each with a lifting frequency of l Iiftlminute.

Using the new concept:

CLI = L1 A•1 + (L1 •. 2 - LI.)

In these equations, the numeric part of the subscript represents the frequency, such that U B) indicates the U value for Task B at a frequency of 2 liftsIminute and U B•I indicates the U value for Task B at a frequency of 1 Iiftlminute.

Since task A and B are identica1, U-'-I and Ila.1 caneel out and CLI = U B). As expected, the CLI for the job is equivalent to the U value for the simple task being perfrnmed at a rate of 2 timeslminute. Now, if the two tasks are different, then

38

In this case, UA,I and U B•I do oot caneel each other out. The Cl1 is equa! to the sum of UA,I' OOich refers to the demand of Task A, and the increment of demand for Task B, with the increment being equa! to the increase in demand when the frequency for Task B is increased from l liftIminute (corresponding to the frequency of Task A) to a rate of 2 liftslminute (corresponding to the sum of the frequencies of Task A and B). Thus, as each additional task is added, the ru is increased apptopIiately.

While the new rnethod has not been validated at the workplace, this multi-task version will minimize errors due to averaging; and thereby, provide a more accmate rnethod for estimating the combined effects of multi-tasked lifting jobs than vws provided in the NIOSH WPG (1981).

Many of the lifting jobs in the workplace have multiple lifting activities, and therefore could be analyzed as either a single or a multi-task lifting job. When detailed inforrnation is needed, however, to specifY engineering rnodifications, then the multi-task approach should be used On the other hand, the multi-task procedure is more complicated than the single-task procedure, and requires a greater understanding of assessment terminology and mathematical concepts. Therefore, the decision to use the single or multi-task approach should be based on: (1) the need for detailed infonnation about a11 facets of the multi-task lifting job, (2) the need for accuracy and completeness of data in perfonning the analysis, and (3) the analyst's level of understanding of the assessment procedures.

To perform a lifting analysis using the revised lifting equation, two steps are undertaken: (1) data is collected at the worksite and (2) the Recorrunended Weight Limit and Lifting Index values are computed using the single-task or multi-task analysis procedure. These two steps are described in the following sections.

39

2.2. CoHect Data (Step l)

The relevant task variables must be carefully measured and clearly recorded in a concise format. The Job Analysis Worksheet for either a single-task ana1ysis (Figure 3) or a multi-task ana1ysis (Figure 4) provides a simple fonn for recording tbe task variables and tbe data needed to calcu1ate tbe RWL and tbe li values. A thorough job ana1ysis is required to identify and catalog each independent lifting task that cornprises tbe worker's complete job. For multi-task jobs, data must be collected for each individuai task The data needed for each task include tbe following:

1. Weigbt of tbe oiject lifted. Detemrine tbe load weight (L) of tbe object (ifnecessary, use a scale). Iftbe weight oftbe load varies frorn lift to lift, record tbe average and maxirnum weights.

2. IlIrizonflll and venicallocatioll'l of tbe bands willi respect ID tbe mid-point between tbe anldes. Measure tbe horizontal location (H) and vertical location (V) of tbe hands at bolli tbe origin and destination.

3. AogIe of zylillIleÙ). Detemrine tbe angle of asymmetry (A) at tbe origin and destination of tbe lift.

4. FrecJiency of lift. Detemrine tbe average lifting frequency rate (F), in lifts/min, periodically iliroughout tbe work session (average over at least a 15-minute period)o Iftbe lifting frequency varies frorn session to session by more than t\\Q

lifts/min, each work session should be ana1yz.ed as a separate task The duration category, however, must be based on tbe overall work pattern of tbe entire workshift.

5. lifting WnmolL Detemrine tbe totaI time engaged in continuous lifting and tbe schedule of recovery allowances (Le., light work assigmnents) for each lifting task Compute tbe recovery-time to work-time ratio to c1assify tbe job for work duration (Le., Short, MxIerate, or Long).

40

JOB ANALYSIS WORKSHEET DI!PARTMI!NT J08 DE8CRIPTION JOII TrrLE

ANALYST'S NAME DATE

STEP 1. Maaaure and racord ta.k varlabla.

Object Hand Localion (in) Vertice.1 Asymmetric Angle (degrees Frequency Rate Duralion Objecl Weight (1bs) Origin De" Oistance (in) Origin Destinalion lilts/min (HRS) Coupling

L (AVG.) L (M(l)(.,) H V H V D A A F C

.j>. ..... STEP 2. Datarmlna tha multlpllar. and computa tha RWL' •

ORIGIN

RWL • LC • HM • VM • DM. AM. FM. CM I RWL·~·CJ·CJ·CJ·CJ·CJ·CJ- Lbe I

DESTINATION RWL .[ill. CJ· CJ· CJ· CJ· CJ· CJ-I Lbe 1

STEP 3. Computa tha LIFTING INDEX

-D LIFTING INDEX -08JECT WEIGHT (L)

ORIGIN -RWL

DESTINATION LIFTING INDEX -OBJECT WEIGHT tl) -D RWL

.. Figure 3: Singie Task Job Analysls Worksheet

MULTI·TASK JOB ANALYSIS WORKSHEET

D!PAATM!NT JDI D!SCAIPTION JOB TITL!

ANALYST'8 NAM!

DATE

STEP 1. • •• ur. an Record ,...k Va,labl_ Da'a

T •• k No. .. ~~ject H.lnd Locatlon (In) V.rtlcal A. mm.t .... , " ". Duralion Couplll'\g W.IgM (I.~ . " •• 1. l,tMce (In) . '" ... III mn H"

Mp H A ,

i!:s 8TEP 2. Computa multipli.,. and FIRWL. STRWL, FILI, and STLI 'or E.eh T •• k

~:~ Le x HM li VM li DM li AM li CM 'IAWL • PM 8TAWL .s.:~~WL S!!;',_ I ÙSTAWl. "'::':.0. F

11

11

11

11

11

IITEP s. Compute the Comlla.lte Uftlng Inde. for the Job (Aft.r Nnumberlna ... ke, CU' 8TLI, + /)"PIU, + A'IU, + A 'IU, + /)" FILI,

PlU.(1/'M .... 1'N,) "LI,('''''' . ,,, .. , , .. C1/' .. • 1/'M l." PlLI",U/'" 01 , .. ~

CU

Figure 4: MULTI·TASK JOB ANALYSIS WORKSHEET

6. Couping type. Oassify the hand-t<HXlntainer coupling based on Table 6

2.3. Single-TlL'ik Assessment (Step 2)

Calcu1ate the RWL at the origin for each lift. For lifting tasks that require significant control at the destination, calcu1ate the RWL at bolli the origin and the destination of the lift. The 1atter procedure is required if (I) the 'OOI'ker has to re-grasp the load near the destination of the lift, (2) the \\\Jrker has to momentarily hold the object at the destination, or (3) the \\UIker has to position or guide the load at the destination. The purpose of calcu1ating the RWL at both the origin and destination of the lift is to identify the most stressful location of the lift. Therefore, the lower of the RWL values at the origin or destination should be used to compute the Lifting Index for the task, since this value \\\Juld represent the limiting set of conditions.

The assessment is completed on the single-task \WI'ksheet by detennining the lifting index (l.l) for the task of interest. This is accomplished by comparing the actua1 weight of the load (L) lifted with the RWL value obtained frorn the lifting equation.

2.4. Multi-TlL'ik ProceWre

l. Compute the Frequency-Independent Recommended Weight Limit (FlRWL) and Single-Task Recommended Weight Lirnit (S1RWL) for each task.

2. Compute the Frequency-Independent Lifting Index (FIll) and Single-Task Lifting Index (STLI) for each task.

3. Compute the Composite Lifting Index (CLI) for the overnll job.

43

2.4.1. Compite tbe F1RWL for F.ach Tll'Ik

Compute the Frequency Independent Weight Limit (FIRWL) value for each task by Il'ling the respective task variables and setting the Frequency Multiplier to a value of 1.0. The FIRWL for each task reflects the comptessive foree and IllU'lCle strength demands for a single repetitiOll of that task. If significant control is required at the destinatiOll for any iD:Ii.viduai task, the FIRWL must be computed at both the origin and the destinatiOll of the Iift, as described above for a single-task analysis.

2.4.2. Co1Iplte tbe SIRWL for F.ach Tll'Ik

Compute the SiogIe-Task Recommended Weight Limit (S1RWL) for each task by muItiplying its FIRWL by its app:opriate Frequency Multiplier (FM). The S1RWL for a task reflects the overall demands of that task, assmning it was the onIy task being perfOI"IDed Note, this value does not reflect the overall demands of the task when the other tasks are considered. Nevertheless, this value is helpful in determining the extent of excessive physical stress for an iD:Ii.viduai task.

2.4.3. Co1Iplte tbe F1U for F.ach Tll'Ik

Compute the Frequency-Independent Lifting Index (Flli) for each task by dividing the tntDCintzm load weight (L) for that task by the respective FIRWL. The maximum weight is Il'led to compute the FII1 becallse the maximum weight determines the maximum biomx:hanicalloads to ~ch the body wilI be exposed, regardless of the frequency of occurrence .. Thus, the FII1 can identify iD:Ii.viduai tasks with potentiaI strength problems for infrequent Iifts. If any of the FII1 values ~ a value of LO, then ergonomic changes may be needed to decrease the strength demands.

44

2.4.4. Compute tbe SIll for Each T~k

Compute the Single-Task Lifting Index (S1ll) for each task by dividing the flIJerage load weight (L) for that task by the respective S1RWL. The average weight is used 10 compute the STLl because the average weight provides a better representation of the metabolic demands, \Wich are distributed across the tasks, rather than dependent on individuai tasks. The STIl can be used 10 identify individuai tasks with excessive physical demands (i.e., tasks that would result in fatigue). The STLl values do not indicate the relative stress of the individuai tasks in the context of the \\baIe job, but the STLl value can be used 10 prioritize the individuai tasks according to the magnitude of their physical stress. Thus, if any of the STLl values exceed a value of LO, then ergonomic changes may be needed 10 decrease the overall physical demands of the task. Note, it may be possible to bave a job in \Wich ali of the individuai tasks bave a STLlless than 1.0 and stili be physically demanding due 10 the combined demands of the tasks. In cases 00ere the Flll exceeds the STLl for any task, the maximum weights may represent a significant problem and careful evaluation is necessary.

2.4.5. Compite tbe 01 for tbe .lIb

The assessment is completed on the multi-task worksheet by detennining the Composite Lifting Index (CLI) for the overall job. The Cl1 is computed as follows:

1. The tasks are renumbered in order of decreasing physical stress, beginning with the task with the greatest STLl down 10 the task with the smallest STIl. The tasks are renumbered in this way so that the more difficult tasks are considered fust.

45

2. The ili for the job is then computed according to the following formula:

CU = STU1 + L I1U Where:

L I1U = (FIU2 X (1 FM1 »

FM1,2 1

+(FIU3 X (1 1» FM FM 1,2,3 1,2

+ (FIU4 X (1 1» FM1,2,3.4 FM1,2.3

+(FIUn X (FM 1 1,2,3.4 •... ,n

1 FM »

1 ,2,3 ••..• (n -1)

Note, that (I) the numbers in the subscripts refer to the new task numbers; ani, (2) the FM values are determined from Table 5, based on the sum ofthe trequeocies for the tasks listed in the subscripts.

46

The following exarnple is provided to demonstrate tIùs step of the multi-task procedt.n-e. Asswne that an analysis of a typical three­task job provided the following results:

Task Number 1 2 3

Load Weight (L) 30 20 IO

Task Frequency (F) I 2 4

FIRWI.. 20 20 15

FM .94 .91 .84

5TRWI.. 18.8 18.2 12.6

FILI 1.5 1.0 .67

5TLI 1.6 1.1 .8

New Task Number I 2 3

To compute the Composite Lifting Index (CLI) for tIùs job, the tasks are renumbered in arder of decreasing physical stress, begimring with the task with the greatest SIU down to the task with the smallest SIU. In tIùs case, the task numbers do not change. Next, the CIl is computed according to the formula shown on the previOI.lS page. The task with the greatest CIl is Task l (SIU = 1.6). The sum of the frequencies for Tasks l and 2 is 1+2 or 3, and the sum ofthe frequencies for Tasks 1,2 and 3 is 1+2+4 or 7. Then, from Table 5, fMt is .94, fMt,2 is .88, and fMt,2,3 is .70. Finally, the CIl = 1.6 + 1.0(1/.88 - 1/.94)+.67(1/.70 - 1/.88) = 1.6 + .07 + .20 = 1.9. Note that the 1M values were based on the sum of the frequencies for the subscripts, the vertical height, and the duration of lifting.

47

3.1. How fii Use tbe EtlllllPe Proliems

There are severa! awroaches for controlling the stressors reIated to manua1lifting. One approach is to eliminate the manua1 re.quirem:nts of the job by ming hoists, cranes, manipu1ators, chutes, conveyors, or Iift trucks, or through mechanization or automatiOIL Ifthe manua1 re.quirem:nts ofthe job cannot be eIiminated, then the demands of the job should be redJ JCed through ergonomic designlredesign (e.g., modify the physical Iayout of the job or reduce the frequency or duration of lifting). As a Iast resort, and if redesign is not feasible, the stress on the \\Orlrer should be redJJCed by distributing the stress between t\\U or more \\Ufkers (e.g., team Iifting).

In many cases eIimination of manua1 lifting is not feasible or prnctical. Thus, ergonomic designlredesign is the best available control strategy. The goal of such a strategy is to reduce the demands of the job by reducing exposure to dangerom loading conditions and stressfuI body movements.

Ergonomic designlredesign iocludes: (l) physical changes in the Iayout of the job, (2) reductions in the lifting frequency rate and/or the duration of the \\Urk period, and (3) modifications of the physical properties of the object Iifted, such as type, size, or \Wight and/or improvement of hand-to-object coupling.

The lifting equation and procedures presented in this docwnent \\ere designed to identify ergonomic problems, and evaIuate ergonomic designlredesign solutions. By examining the value of each task muItiplier, the penalties associated with each job-reIated risk factor can be evaIuated, thereby determining their relative importance in consideration of alternate \\Urlq>lace designs. The task factors that = the greatest reduction in the 1000 constant should be considered as the first priority for job redesign.

48

Ten exarnples are provided to demonstrate the proper application of the lifting equation and procedw-es. The procedures provide a method for deterrnining the level of physical stress associated with a specific set of lifting conditiOlJS, and assist in identifying the contribution of each job-related factor. The exarnples also provide guidance in developing an ergonomic redesign strategy. Specifically, for each exarnple, a job description, job analysis, hazard assessment, redesign suggestion, illustration, and completed \\QI"ksheet are provided. The ten exarnples were chosen to provide a representative sample of lifting jobs for \\hlch the application of this equation was suitable.

Note, you might obtain slightly different values from those displayed in the \\QI"ksheet exarnples due to differences in rounding, especially when these values are compared to those determined from computerized versions of the equation. These differences should not be significant. Also, for these exarnples, multipliers are rounded to two places to the right of the decimai and weight limit (RWL, FIRWL, and S1RWL) and lifting index values (Il, FlLI, Sru, and CLI) are rounded to one piace to the right of the decimalo

1he exarnples are organized as follo-.w:

A Single Task, Performed a Few Tunes Per Shift Loading Punch Press Stock, Example l Loading Supply Rolls, Example 2 Loading Bags Into A Hopper, Example 3

B. Single Task, Performed Repetitiveiy Package Inspection, Example 4 Dish-Washing Machine Unloading, Example 5 Product Packaging I, Example 6

C. Multi-Task, Short Duration (1 hr or less) Depalletizing Operation, Example 7 Handling Cans of Liquid, Example 8

49

D. Multi-Task, Long Duration (more than 2 hours but less than 8) Product Packaging n, Example 9 Warehou<;e Order Filling, Example lO

To help clarify the discussion of the lO example probleJm, and to provide a u<;eful reference for detennining the muItiplier values, each of the six nruItipliers used in the equation bave been reprinted in tabular fonn in Tables 1 tbrough 5 and Table 7 on the following page.

50

Table 1 Table 2 HorizontaI Multiplier Vertica1 Multiplier

H HM H IHM

" an

1"'0 ,.~ >Q 1.00

11 . ., iO> I·" '2 ... ""- .83

13 .T1 32 .IO

" .71 34 .7.

,. .., .. ... IO . .., .. .66

" ~ ""- . .., 'O -'" 42_ l:"" ,. .03 ," 1." m .~ .. . 54 ., ."" "" .0<

22 ... L"'. I·'" '" ... 02. L"" 24 .42 54 I·" "" ... ,"" 1.45

>"" .~ "" ... , "" I·'" , "'. L'4U

1>63 .00

Table3 Distance Multiplier

u I .... u ... " un

,,",o '.00 1=- '.00 '5 ... 40 .93

1

m ... 00 . 90

l'''' ... 'O .58 ,.., ... ... .D'

l"" .. , ,w_ c"'. 140 .01 " . .... 145 .... 130 .... I~ .00 ,~ .55

lO. .00 160 .55

l'''' .... IO .00

70 ... >1f' -='" >70 .00

v

" O

• .1U , .

120

l'''' l''' l"" L'" l'''' 1

50

lO. l'''' 100

IO

>70

VM V VM

un .78 O .70

.., 'v .• , ... -'" ...

.... "" ' .. " .93 40 .90

.~ 50 .93

,.~ "" ,'~ ... 'v I·" .,., ou ... .... YU ,"" .55 100 1.93 .. , 10V .90

"D ,.'V , .• , "4 ,.., I·" .fU 140 .8,

.00 '50 .f0

160 .75

HV ... 1 5 .'v

>HO CW

Table 4 Asyrnmetric Multiplier

A /Wl

""" v 1.~

lO c""-.., .90

45 .... 60 .01 ,. .,. w .71

,v> ... '20 ."" ,JO .51

>lJO .00

51

F .... "*'

l""

" , 2

• • • • I

O

• 'v

"-1Z

'3 ,. , . >",

Table 5 Frequency Multiplier

<,o... , ......... ......... ve v?, ve v?, ve ,v?,

30 in 30 in 30 in 30 in 30 in 30 in

1'·00 1,·00 ... ... .... I·" . ., :!'. . .., .:"'. .. , ':'" ... ... ... ... .75 .I, .. , .• , .54 .54 .85 .55 ... ... .,. .79 .55 1.55 ... .84 ... . .. .40 .40

. ... .DV ."" ."" ... ... .15 J' .00 -,'" . ., L'" .70 .lU .42 .... Z2 I .<2

.60 .60 .JO .JO .18 .'0

. 0< • 0< .... .30 .00 . , .

.40 . 40 ... .." .00 .1 •

," -="- -"" ... • 00 ,.w .37 . ., .00 ", .00 1.00

.00 .34 .00 .00 .00 1.00

.~ .31 .00 .00 .00 .00

. 00 .<D .~ .~ .~ .~

.00 .00 ____ w

.00 .00 .W

Table 7 Coupling Multiplier

COU'UNG ....

TVPE ve in in ........, ,.~ ,.w

."'" ... ,.~

~ .w .YU

A series of generai designlredesign suggestions for each job-related risk factor are provided in Table 8. These suggestions can be used to deve10p a practical ergonomic designlredesign strategy.

TaHe8 Geneml Design'Redesign SuggestiOIlS

If HM is less Bring the load closer to the worker by than 1.0 removing any horizontal barriers or reducing

the size of the object. lifts near the floor should be avoided; if unavoidable, the object should fit easily between the legs.

IfVM is less Raise/lower the origin/destination of the lift. than 1.0 Avoid lifting near the floor or above the

shoulders.

If DM is less Reduce the vertical distance between the than 1.0 origin and the destination of the lift.

If AM is less Move the origin and destination of the lift than 1.0 closer together to reduce the angle of twist,

or move the origin and destination further apart to force the worker to turn the feet and step, rather than twist the body.

If FM is less Reduce the lifting frequency rate, reduce the than 1.0 lifting duration, or provide longer recovery

periods (i.e., light work period).

If CM is less Improve the hand-to-object coupling by than 1.0 providing optimal containers with handles or

handhold cutouts, or improve the handholds for irregular objects.

If the RWL at Eliminate the need for significant control of the the object at the destination by redesigning destination the job or modifying the container/object is less than characteristics. (See requirements for at the origin significant control, p. 36, 43.)

52

3.2 • .llbs PeIfonned a Few Thnes Per Sbift

3.2.1. Loading Punch Press Stock, ExlllDPe l

3.2.1.1 • .llb Description

Figure 5 iIlustrates a common oversight in physica1ly stressful jobs. A pw1Ch press operator routinely handles small parts, feeding them into a press and removing them. A cursory view of thls task may overlook the fact that once per shift the operator must load a heavy reel of supply stock (illustrated at floor height) from the floor onto the machine. The diameter of the reel is 30 inches, the width of the reel between the worker's hands is 12 inches, and the reel weighs 44 Ibs. Significant control of the load is required at the destination of the lift due to the design of the machine. Also, the worker cannot get closer to the roll (i.e., between the legs) because the roll is too awkw.nd

3.2.1.2. .llb Analysis

The task variable data are measured and recorded on the job analysis worksheet (Figure 6). Asswning the operator lifts the reel in the piane shown, rather than on the side of the machine, the vertica1 height (V) at the origin is 15 inches, the vertica1 height (V) at the destination is 63 inches, and the horizontal distance (lI) is 23 inches at both the origin and the destination of the lift. The activity occurs only once per shift, so F is assmned to be < 0.2 (see Table 5), and duration is assmned to be less than I bour.

No asymmetric lifting is involved (i.e, A = O), and according to Table 6, the couplings are classified as fair because the object is irregular and the fingers can be flexed about 90 degrees. Since significant control is required at the destination, the RWL must be computed at both the origin and the destination of the lift.

53

H OESTINATION ANKLELOCA~I _____ .~~----~Si~Esi----' ASSUMES 23 INCHES STEP FORWARD

WfTH LOAD

I HORIGIN

• 23 INCHES

V ORIGIN 15 INCHES

YOESTINATION 63 INCHES

Figure 5 Loading Punch Press Stock, Exarnple 1

54

The multipliers are detennined from the lifting equation or from tables (fables l to 5, and Table 7) .. The CM is .95 at the origin and 1.0 at the destination, due to the difference in the vertical height at the origin and destination. As shown in Figure 6, the RWL for this activity is 16.3 Ibs at the origin and 14.5 Ibs at the destination.

3.2.1.3. Hrt.anl Assessment

The weight to be lifted (44 lbs) is greater than the RWL at both the origin and the destination ofthe lift (16.3 lbs and 14.5 Ibs, respectively). The Il at the origin is 44/16.3 or 2.7, and the Il at the destination is 44/14.5 or 3.0. These values indicate that this lift would be hazardous for a rnajority of healthy industriai workers.

3.2.1.4. Redesign Suggestions

The worksheet shown in Figure 6 indicates that the srnallest multipliers (i.e., the greatest pena1ties) are .44 for the HM, .75 for the VM at the destination, and .86 for the DM Using Table 8, the following job modifications are suggested:

1. Bring the object closer to the worker at the destination to increase the HM value.

2. Lower the destination of the lift to increase the VM value.

3. Reduce the vertical travel distance between the origin and the destination of the lift to increase the DM value.

4. Modify the job so that significant control of the object at the destination is not required This will eliminate the need to use the lower RWL vaIue at the destination.

1f the operator could load the machine from the side, rather than the from the front, the reel could be tumed 90" whlch would reduce the horizontal location of the hands at the origin

55

JOB ANALYSIS WORKSHEET DEPARTIIENT Manufacturing Joa DEIICRIPTION J08 TITLE Punch Presa DEerator Loading: sum211 stock onto ANALY8T'8 MAME 12unch I;!ress machine DATE Examole I

STEP 1. Maaaura and racord ta.k varlabla.

Object Hard location (in) Vertical Asymmelric Angle (degrees Froquency Rate pUfl!ltion Ob;ect We~ht (lbI) . Origin Oest Dislance (in)

" sin IOn Il mln Coupling

L M"

44 I 44 23 115 23 1 63 48 O O <.2 <I Fair

~ STEP 2. Datarmlne tha multlpller. end computa tha RWL'. RWL = LC • HM • VM • DM. AM. FM. CM

OAIGIN RWL =[ill·[]!)·mJ·mJ·II:Q]·~·~ = 1163 Lb. I DESTINATION RWL =[ill. cm· ~.[!!].II:Q]. ~. ~ = 1145 Lb.

1

STEP 3. Computa tha LIFTING INDEX

=--iL-=~ LIFTING tNDEX .. 08JECT WEIGHT (L)

OAIGIN RWL 16.3

DESTINATION LIFTING INDEX -OBJECT WEIGHT (L) =~=EJ RWL 14.5

Figure 6: Example 1, Job Analysls Worksheet

(Le., H = lO inches) and destination of the lift (Le., H = 12 inches). The grip, ho~, wouId be poor because the object is buIky and hard to handle and the fingers couId not be flexed near 9(jl \\hm picking up the reel (see Table 6, Note 4).

The RWL and corresponding li vaIues for this preferred combination of task variables (Le., loading the machine from the side) are shown on the modified job anaIysis sheet (Figw-e 7). At the origin, the RWL is 35.1 Ibs and the li is 1.3. At the destination, the RWL is 24.6 lbs and the li is 1.8. Since the li is still greater than LO, ho\WVer, a more comprehensive solution may be needed. This couId include: (l) lowering the vertical height of the destination, which wouId increase the VM and the DM at both the origin and the destination of the Iift; (2) reducing the size andIor ~ight of the supply reel; or, (3) transferring the supply reel from the storage area on a mobile, mechanical lifting device or jack that couId be moved near the machine to eliminate the need for manuallifting. If it is not feasible to eliminate or redesign the job, then other measures, such as assigning two or more workers, couId be considered as an interim control procedure.

3.2.1.5. Commem

Although ergonomic redesign is preferred, this example demonstrates how a change in work practices (Le., insuring that the operator can load the reel from the side) can reduce the magnitude of physical stress associated with a manual Iifting task. This approach, ho\WVer, relies more on worker compliance than on physical job modifications.

57

VI 00

JOB ANALYSIS WORKSHEET DI!PARTMI!NT Ml!IlufacturlnS! .1011 De:8CRIPTION

.101 TITU Punch Press O;t!:rator Loading sUJ2121I stock onte ANALYST8 NAMI! Qunch Rress machine DAT! M2d1ftgg :E:umJ21g 1

STEP 1. Meaaure and racord taak varlabla.

Object Hanel Locatlon (in) Vertlcel Aeymmetrlc Angle (degre.s Frequency Rale purelion Ob)eCI Weight (Ibe) Origln DasI. Distanee (in) " , sin I0I"l 18 mln Coupllng

'" 44 I 44 IO 116 121 63 48 O O <.2 <1 POOI

STEP 2. Determlne the multlpllera and compute tha RWL'a RWL = LC • HM • VM. DM. AM. FM. CM

ORIGIN RWL =[!IJ. [TI]. rn:J. rn:J ·IITI·IITI· ~ -1 36.1 Lb. I DI!8TINATION RWL =!!!J.~. [!!l. ~ ·IITI· lITI . ~ -124.6 Lb.

1

STEP 3. Computa tha LIFTING INDEX

LIFTING INOEX _ OBJeCT WEIGHT (L) a~-G2J ORIGIN RWL 36.1

DI!8TINATION LIFTING INOEX _ OBJECT WEIGHT (L) =~-EJ RWL 24.6

Figure 7: Modlfled Example 1, Job Analysls Worksheet

3.2.2. lnading SURiY RoUs, Example 2

3.2.2.1 • .lIb Descripion

With both hands directly in front of the body, a \\Ul"k:er Iifts the core of a 35-lb roll of paper from a cart, and then shifts the roll in the hands and holds it by the sides to position it on a machine, as shown in Figure 8. Significant control of the roll is required at the destination of the Iift. AIso, the worker must crouch at the destination of the Iift to support the roll in front of the body, but does not bave to twist.

3.2.2.2. .lIb Analysis

The task variable data are nrasured and recorded on the job anaIysis \\Wksheet (Figure 9). The verticaI location of the hands is 27 inches at the origin and IO inches at the destination. The horizontaI location of the hands is 15 inches at the origin and 20 inches at the destination. The asymmetric angle is O degrees at both the origin and the destination, and the frequency is 4 IiftsIshift (i.e., less than .2 liftslmin for less than 1 hour - see Table 5).

Using Table 6, the coupling is c1assified as poor becal.l'le the \\Ul"k:er must reposition the hands at the destination of the Iift and they can not flex the fingers to the desired 9(jl angle (e.g., hook grip). No asymmetric lifting is involved (i.e., A = O), and significant control of the object is required at the destination of the Iift. Thus, the RWL should be computed at both the origin and the destination of the Iift. The nrultipliers are computed from the lifting equation or determined from the nrultiplier tables (Tables 1 to 5, and Table 7). As shown in Figure 9, the RWL for this activity is 28.0 Ibs at the origin and 18.1 Ibs at the destination.

59

20 _ INCHES

I 15 10 INCHES

I

F1gure 8 Loading Supply RoIls, Exarnple 2

60

JOB ANALYSIS WORKSHEET DEPARTMENT ShiEEinll JOB DE8CRIPTION

JOB TITLE Packag:er Loading: l2a12er SU12121;t rolIs ANALY8T'8 NAME DATE Example 2

STEP 1. Me •• ura and racord ta.k varlabla.

Object Hand Location (in) Vertical Asymmetric Angle (degrees Frequency Rate Duration Objec! Weighl ~bs) Origin Desl. Distance (in) flgln stmstion Il mln Coupling

L Max.

35 I 35 15 27 20 IO 17 O O <,2 <I Paor

0\ ..... STEP 2. Datarmlna tha multlpllar. and computa tha RWL' • RWL = LC • HM • VM • DM. AM. FM. CM

ORIGIN RWL =[ill. ffil· rn:J. illJ ·1TIl·1TIl . ~ = I 28, O Lb. I DESTINATION RWL =WJ· [M]. Wl· cm· [LQ]. WJ· [9QJ = 118,1 Lb. I STEP 3. Computa tha LIFTING INDEX

=~ OBJECT WEIGHT (l) 35 ORIGIN LIFTING INDEX - - 28,0 RWL

DESTINATION LIFTING INDEX -OBJECT WEIGHT (l)

= 1~51 =~ RWL

Figure 9: Example 2, Job Analysls Worksheet

3.2.2.3. HazanI AssessmeDt

1he \\eight to be lifted (35 lb) is greater than the RWL at both the origin and destination of the lift (28.0 lb and 18.1 lb, lespectively). 1he li at the origin is 35 100128.0 lbs or 1.3, and the li at the destination is 35 lbsl18.1 lbs or 1.9. 1hese vaIues indicate that this job is only sligbtly stressful at the origin, but mxlemtely stressful at the destination ofthe lift.

3.2.2.4. Redesign Suggestiom

1he first choice for reducing the risk of injury for \WIkers peIforming this task \\OOld be to adapt the cart so that the paper rolls could be easily pusbed into position on the machine, without manually lifting them.

lfthe cart cannot be modified, then the resuIts ofthe equation may be used to suggest task modificatiom. 1he \WIksheet displayed in Figure 9 indicates that the nrultipliers with the smallest magnitude (i.e., those providing the greatest penalties) are .50 for the HM at the destination, .67 for the HM at the origin, .85 for the VM at the destination, and .90 for the CM vaIue. Using Table 8, the foIlowing job modificatiom are suggested:

1. Bring the load closer to the \\UIker by making the roIl smaller so that the roIl can be lifted from between the workeI's legs. This will decrease the H vaIue, ~ch in tum will increase the HM value.

2. Raise the height of the destination to increase the VM.

3. Improve the coupling to increase the CM.

lf the size of the roIl can not be rerlurel, then the vertical height (V) of the destination should be increased. Figure lO show.; that if V was increased to about 30 inches, then VM \\OOld be increased from .85 to 1.0; the H value \\OOld be decreased from 20 inches to 15 ioches, \\hich \\OOld increase HM from .50 to .67.; the DM

62

\Wuld be increased from .93 to 1.0. ThU'), the fina! RWL \Wuld be increased from 18.1 Ibs to 30.8 Ibs, and the il at the destination \Wuld decrease from 1.9 to 1.1.

In some cases, redesign may not be feasible. In these cases, l.L')e of a mechanical lift may be more suitable. As an interim control strategy, t\W or more \WI"kers may be assigned to lift the supply roll.

3.2.2.5. Comments

The horizontal distance (H) is a significant factor that may be difficuit to reduce because the size of the paper rolls may be fixed. Moreover, redesign ofthe machine may not be practical. Therefore, e1imination of the manua1 lifting component of the job may be more appropriate than job redesign.

63

JOB ANALYSIS WORKSHEET DI!P"flTMI!NT Shi22inll JOB DEICRIPTION

JOB TITLE Packaller Loading RaRer BURRly rollB ANALY8TI HAll! DATI! Flodilied ~xamp!. ~

STEP 1, Meeeure .nd record teek verleble.

Objecl Hanel Location Qn) Vertlcal Asymmetrlc Angle (degrees FreQuency Rate Durallon ObjeCI Welght Qbs) Orlgln DeSl Distanee (In) Origin Destinatlon lills/min (HRS) CoupHng

L (AVG,) l (Max,) H V H V D A A F C

36 I 36 16127 16 I 30 3 O O <.2 <I Poor

~ STEP 2, Determlne the multlpllere end compute the RWL'. RWL .. LC • HM • VM • DM. AM. FM • CM

ORIOIN RWL =[ill. [ill.~. [ITJ. [TI]. [ITJ ·1.90 1-1 30.1 Lbo I DESTINATION RWL .. [ii]. Wl· [ll). [ll]. [j]J. [ll). cag] .. 130.8 Lbo

1

STEP 3, Compute the LIFTING INDEX OBJECT WEIGHT (L) 36 ~ ORIOIN LIFTING INDEX - • 3lJ.T"''' l. 2 RWL

DESTINATION LIFTING INDEX -OBJECT WEIGHT (L) = 3~68 =G RWL

Figure 10: Example 2, Modlfled Job Analysls Worksheet

3.2.3. Lnading ~ Jnto A Hower, Ex~e 3

3.2.3.1 • .lIb Description

The 'MlI"ker positions hiImeIf midway between the handtruck. and the mixing hopper, as iIlustrated in Figure Il. Without moving bis feet, he twists to the right and picks up a bag off the handtruck. In one continuous motion he then twists to bis left to piace the bag on the rim ofthe hopper. A sharp edged biade within the hopper cuts open the ba.g to alIow the contents to fall into the hopper. 1his task is done infrequently (i.e., 1-12 times per shift) with large recovery periods between lifts (i.e., > 1.2 Recovery TurelWork Ttme ratio). In observing the 'MlI"ker perfonn the job, it was detennined that the non-lifting activities could be disregarded because they require minima.l force and energy expenditwe.

Significant control is not required at the destination, but the worker twists at the origin and destination of the lift. Although severa! ba.gs are stacked on the band truck, the bighest risk of overexertion uyury is associated with the bag on the bottom of the stack; therefore, only the lifting of the bottom ba.g will be examined. Note, ho~, that the frequency multiplier is based on the overall frequency of lifting for ali of the bags.

3.2.3.2. .lIb Analysis

The task variable data are measured and recorded on the job analysis 'MlI"ksheet (Figure 12). The vertical location of the hands is 15 inches at the origin and 36 inches at the destination. The horizontal location of the hands is 18 inches at the origin and lO inches at the destination. The asymmetric angle is 45" at the origin and 45" at the destination of the lift, and the frequency is less than .2 lifts/min for less than 1 hom (see Table 5).

65

ASYMMETRY UNE Al OAIOIN

.... GITTAL UNE

Figure 11 Loading Bags Into Hopper, Example 3

66

Using Table 6, the coupling is classified as fair because the \\UI'ker can flex the fingers about 9ff' and the bags are semi-rigid (i.e., they do not sag in the middle). Significant control of the object is not required at the destination of the lift so the RWL is computed on1y at the origin The multipliers are computed frorn the lifting equation or detennined frorn the multiplier tables (Tables l to 5, and Table 7). As shown in Figme 12, the RWL for this activity is 18.9Ibs.

3.2.3.3. Hmud AssessmeDt

The ~ight to be lifted (40 Ibs) is greater than the RWL (18.9 Ibs). Therefore, the Il is 40/18.9 or 2.1. This job \Wuld be physically stressful for many incItmial \\UI'kers.

3.2.3.4. Redesign SuggestiOIl!l

The worksheet shows that the sma1Iest multipliers (i.e., the greatest penalties) are .56 for the HM, .86 for the AM, and .89 for the VM Using Table 8, the following job nxxIifications are suggested:

1. Bringing the load closer to the worker to increase the HM.

2. Reducing the angle of asynnnetry to increase AM This could be acromplisbed either by rnoving the origin and destination points closer together or further apart.

3. Raising the height at the origin to increase the VM

If the \Wlker could get closer to the bag before lifting, the H value could be decreased to lO incbes, which \Wuld increase the HM to LO, the RWL \Wuld be increased to 33.71bs, and the Il \Wuld be decreased to 1.2 (i. e., 40133.7).

67

JOB ANALYSIS WORKSHEET DEPARTMENT Manufacturin!i! Joa DEeCAIPTION .IO. TITL~ Batch Processor Cumglcg bags i:ctc mbdcg ANALV'ST'e NANE hOEEer DAr. ExamEle 3

STEP 1. Meeeure end record teek verleblee

Cblsel Hand LocaHon (In) Venlcal Asymmetrlc A!'\gle (degrees Frequency Rate Duratlon Objeci Welghl (Iba) Or In Deal. Dlslance (In) Orlgln DestinellOn IIIIa/mln (HAS) CoupUng

L (AVG,) L (Max.) H V H V D A A F C

40 T 40 18 18 IO f36 21 45 48 <.2 <I Fair

&l STEP 2. Oetermlne the multlpllere end compute the RWL'. RWL = LC • HM • VM • DM. AM. FM. CM

ORIGIN RWL =IliJ. m:J. ~. cm· rnJ· [[§J. [ID -1 18.9 Lbo I DESTINATION RWL =lliJ.D'D'D'D.D.D-! Lbo !

STEP 3. Compute the LIFTING INOEX OBJECT WEIGHT (L) 40 ~ ORIGIN LIFTING INDEX .. = '"ì!T" 2.1 RWL

DESTINATION LIFTING INDEX .. OBJECT WEIGHT (l) =-=D RWL

Figure 12: Example 3, Job Analysls Worksheet

3.2.3.5. Commem

This example dernonstrates that certain 1ifting jobs may be eva1uated as a single-task or nru1ti-task job. In this case, on1y the most stressful component of the job was eva1uated. For repetitive lifting jobs, the nru1ti-task approach may be more awlOpIÌate. (See Examples 7-10).

3.3. Single Task, Perfonned Repetitively

3.3.1. Package Impection, F;xample 4

3.3.1.1 . .bb Description

The job illustrated in Figme 13 consists of a worker inspecting compact containers for damage on a low shelf, and then lifting them with both hands directly in front of the body from shelf l to shelf 2 at a rate of 3/min for a duration of 45 rninutes. For this anatysis, asswne that (1) the \\Urker cannot take a step forward when placing the object at the destination, due to the bottom shelf, and (2) significant control of the object is required at the destination. The containers are of optimal design, but without handles (For c1assification, refer to Table 6).

3.3.1.2 • .bb Anatysis

The task variable data are measured and recorded on the task anatysis worksheet (Figme 14). The horizontal distance at the origin ofthe lift is lO inches and the horizontal distance at the destination of the lifi is 20 inches. The height of shelf one is 22 inches and the height of shelf t\\U is 59 inches. Since the container is of optimal design, but does not have handles or handhold cutouts, the coupling is defined as "fair" (see Table 6). No asymmetric lifting is involved (i.e., A = O). Significant control of the load is required at the destination of the lift. Therefore, the RWL is computed at both the origin and the destination of the lifi.

69

VERTlCAL (+) ASSUME SIGNIACANT

CONTRO!. RECUIRED

.----j-----, j r::::::::+==1F SHELF 2

SII INCHES

SHELF 1

22

Ngure 13 Package Inspection, Exarnple 4

70

, JOB ANALYSIS WORKSHEET

DEPARTMENT çualil1 Control .108 DEBCRIPTION

.I0B TlTLE Packinll Insl2ector Ins};!ect 12ackages ANALYBT'8 NAME DATE Exam121e 4

STEP 1. Meaaure and record lII.k varlable.

Objecl Hand locatlon ~n) Vertlcel Asymmetr1c Angla (degrees Frequency Rate Durallon Object Welght Qbs) Orlgln OasI. Dlstance (In) Or In DestlnaUon IIftstmln I!HRS Caupllng

L (AVG.) L Mox H V H V D A A F C

26 1 26 IO 122 20 159 31 O O 3 .15 Fair

-...l - STEP 2. Oetermlne the multlpller. and compute the RWL'. RWL = LC • HM • VM • DM. AM. FM • CM

ORIGIN RWL =[ill. W·lliJ· [ill. [li] . WJ· [ili = 134.9 lb. I DESTINATION RWL =Cill' [W. cm· cm . [QJ • [ill. W = 115.2 lb. I STEP 3. Compute the LIFTING INOEX

08JECT WElGHT (l) 28 Q ORIGIN LIFTING INOEX _ ="""3T.9" = .8 RWL

DESTINATION LIFTING INDEX _ OBJECT WEIGHT (L)

26 0 RWL =""15T = 1.1

Figure 14: Example 4, Job Analysls Worksheet

The nrultipIiers are computed fiom the lifting equation or detennined fiom the multiplier tables (fables l to 5, aOO Table 7). As shown in Figure 14, the RWL for this activity is 34.9 Ibs at the origin aOO 152 Ibs at the destination

3.3.1.3. Hmud Assessment

The weight to be Iifted (26 Ibs) is less than the RWL at the origin (34.9Ibs) but greater than the RWL at the destination (15.2 lbs). The Il is 26134.9 or .76 (rouOOed to .8) at the origin, aOO the Il is 261152 or 1.7 at the destination These values indicate that the destination of the Iift is lOOre stressful than the origin, aOO that SOIDe reaIthy \WI'kers v.ooId find this task physically stressful.

3.3.1.4. Redesign Suggestiom

The \\UI"ksheet illustrated in Figure 14 shows that the multipIiers with the smallest magnitude (i.e., those that JXOVide the greatest penalties) are .50 for the HM at the destination, .78 for the VM, .87 for the DM, aOO .88 for the FM at the destination of the Iift. Using Table 8, the following job IDJdificatiom are suggested:

1. Bring the destination point closer to the \\Ul"ker to increase the HM value.

2. Lower the height of shelf 2 to increase the VM value.

3. Decrease the vertica1 distance bet\\eell origin aOO destination of Iift to increase the DM value.

4. Reduce the lifting frequency rate to increase the FM value.

5. M:xIify the task so that there in no need for significant control of the object at the destination to eliminate the lower RWL value.

Practica1 job IDJdificatiom could include bringing shelf 2 closer to the \\Ul"ker to reduce li, raising the height of shelf l to increase the

72

CM value, lowering tbe height of shelf 2 to reduce D, or reducing tbe need for significant control at tbe end of tbe lift by providing a receiving chute.

3.3.1.5. Co_nIs

Since tbe lifting pattem is continuous over tbe 45 minute \\crk session, tbe lifting frequency is not adjusted using tbe special procedure described on page 27.

3.3~ Dish-WLoibing Mtchine UnlOldng, Exanl(le 5

3.3~1 . .lIb Descripion

A worker manually 1ifts trays of clean dishes from a conveyor at tbe end of a dish washing machine and loads thern on a cart as shown in Figw-e 15. The trays are filled with assorted dishes (e.g., gIasses, plates, bowls) and silverware. The job takes ~ 45 rninutes and I hom to complete, and tbe lifting frequency rate averages 5 liftsImin Workers usua1ly twist to ODe side of their body to lift tbe trays (i.e., asymmetric lift) and tben rotate to tbe otber side of tbeir body to lower tbe trays to tbe cart in ODe

srnooth continuous motion. The maxinnnn amount of asymmetric twist varies ~ workers and within \\crkers, however, there is usua1ly equa! twist to either side. Dwing tbe lift tbe \\crker may take a step toward tbe cart. The trays bave well designed handhold cutouts and are made of lightweight materials.

3.3.2.2. .lIb Analysis

The task variable data are rneasured and recorded on tbe job analysis \\crksheet (Figw-e 16). At tbe origin of tbe Iift, tbe horizontal distance (H) is 20 inches, tbe vertical distance (V) is 44 inches, and tbe angle of asymrnetry (A) is 3<1'. At tbe destination of tbe lift, H is 20 inches, V is 7 inches, and A is 3<1'. The trays norma1ly weigh from 5 Ibs to 20 Ibs, but for tbis example, assmne that ali of tbe trays weigh 20 Ibs.

73

:i>!

20lNCHES

20lNCHES ~

.. ~ ..

.44 INCHES

ORIGIN

I 30'

SAGITAL LlNE ) ~

/ DESTINATION 30' '----- 71NCHES

/ Figure 15 Dish-Washing Machine Unloading, Exarnple 5

JOB ANALYSIS WORKSHEET DfPARTlIl!NT Food Service .101 DE8CRIPTION 0108 TITLE Cafeteria Worker Unloading a dish-washing ANALvsrl NAMI! machine DATI! Examl2le 5

STEP ,_ Maa.ura and racord taak varlabla.

ObJee! Hand Location Qn) Venlcal Asymmetric Angle (degrees Frequency Rale Duration Object Welghl (Ibs) Orlgln Oesl Distance (in) Orlaln Destination IiflS/mln (HAS) Coupllng

l (AVG.) L Max,) H V H V D A A F C

20 20 20144 20 T 7 37 30 30 5 < l Good I

ì:ìl STEP 2_ Datarmlna tha multlplIar •• nd computa tha RWL'. I

RWL = LC • HM • VM • DM. AM. FM. CM ORIGIN RWL =[ill. []QJ. mJ· [ili. mJ· [ill. [Q] = 114.4 Lb. I DESTINATION RWL =[ill. []QJ.~. [ili.I]Q]. []Q]. [Q] = 113.3 Lb.

1

STEP 3_ Computa tha LIFTING INDEX OBJECT WEIGHT (L) 20 [;]

ORIGIN LIFTING INDEX - =14.4 = 1.4 RWL

DESTINATION LIFTING INDEX -OBJECT WEIGHT (l) 20

=0 ,

= RWL 13.3 !

Figure 16: Example 5, Job Analysls Worksheet

Using Table 6, tbe coupling is cIassified as Glod. Significant control is required at tbe destination of tbe lift. Using Table 5, tbe FM is determined to be .80. As shown in Figure 16, tbe RWL is 14.4 Ibs at tbe origin and 13.3 Ibs at tbe destination.

3.3.2.3. Hazard Assessment

The weight to be lifted (20 Ibs) is greater than tbe RWL at both tbe origin and destination oftbe lift (14.4Ibs and 13.3 Ibs, xespectively). The U at tbe origin is 20114.4 or 1.4 and tbe li at tbe destination is 1.5. These resuIts indicate that this lifting task \\OOld be stressful for some \\UIkexs.

3.3~4. Redesign Suggestiom

The \\UI"ksheet shows that tbe smaIlest muItipliexs (i.e., tbe greatest penalties) are .50 for tbe HM, .80 for tbe FM, .83 for tbe VM, and .90 for tbe AM Using Table 8, tbe following job uxxI.ifications are suggested:

l. Bring tbe load cIoser to tbe worker to increase HM.

2. Reduce tbe lifting frequency xate to increase FM

3. Raise tbe destination of tbe lift to increase VM

4. Reduce tbe angIe of twist to increase AM by either moving tbe origin and destination cIoser together or moving them fwtber apnt. Since tbe horizontal distance (Il) is depeOOent on tbe width of tbe tray in tbe sagittaI pIane, this variable can onIy be redllced by using smaIler trays. Both tbe DM and VM, however, can be increased by lowering tbe height of tbe origin and increasing tbe height of tbe destinatiOIL For example, if tbe height at both tbe origin and destination is 30 inches, tben VM and DM are LO, as shown in tbe uxxI.ified \\UI"ksheet (Figure 17). Moreover, iftbe cart is moved so that tbe twist is e!iminated, tbe AM can be increased from .90 to 1.00. As

76

shown in Figure 17, with these redesign suggestions the RWL can be increased from 13.3 Ibs to 20.4 Ibs, and the il values are rednced to l.0.

3.3.2.5. OJDlIDeDls

This ana1ysis was based on a one-hour \\UI"k session. If a subsequent \Wl"k session begins before the appropriate recovery period has e1apsed (Le., l.2 hours), then the eight -hour category wou1d be used to compute the FM value.

As in the previous example, since the lifting pattern is continwus over the full duration of the \\UI"k sample (Le., more than 15 minutes), the lifting frequency is not adjusted using the special procedure described on page 27.

n

DI!PARTMENT Food Service Joe TrTLI! Cafeteria Worker ANALVS,... NAM~ DATI

STEP 1, Ma •• ura end racord tesk vsrlebls.

Object Hand locatlon ~n) Vertical Weighl (lbs) Orlgln Oesl. Dlslance

) L (Max,) H V H V D A A F \ -"-20 I 20 20130 201 30 O I O I O I 5 I < Il Good

òil I STEP 2, Datarmlna tha multlpllar. snd computa tha RWL'. RWL .. LC • HM • VM. DM. AM. FM. CM r-----,

OHIGIN RWL .. [ill. 00· [TI]. [QJ. [TI]. []Q]. [[] "\20.4 Lbo

DI!STINATION RWL .. [!D. []Q]. [TI]. [QJ. [QJ. []Q]. [QJ .. 120.4 Lbo

STEP 3, Computa tha LIFTING INDEX

ORIGIN LIFTING INDEX -

DI!STINATION LIFTING INDEX -

7: 5,

OBJECT WEIGHT (L)

RWl

OBJECT WEIGHT (L)

RWl

20 .. ~ 20

-2QT

.[;]

.. 0

3.3.3. Procb:t Packaging J, Exan1Jle 6

3.3.3.1. llb Descriplion

In the job illustrated in Figure 18, products \\cighing 25 Ibs arrive via a conveyor at a rate of l-per minute, where a worker packages the product in a cardboard box and then slides the packaged box to a conveyor behind table B. Asswne that significant control of the object is not required at the destination, but that the \\Ufker twists to pick up the product; a1so ~ that the worker can flex the fingers to the desired 9(f angle to grasp the container. The job is perfonned for a nonna! 8-hour shift, including regular rest allowance breaks.

3.3.3.2. llb Analysis

The task variable data are measured and recorded on the job ana1ysis worksheet (Figure 19). At the origin, the vertica1location (V) is 24 inches and the horiwntallocation is 14 inches. At the destination, the vertica1 location is 40 inches, which represents the height of table B plus the beight of the box, and the horizonta1 location is 16 inches.

Using Table 6, the coupling is c1assified as fair. The worker twists 9(f to pick up the product. The job is perfonned for an 8-hour shift with a frequency rate of I-lift per minute. Using Table 5, the FM is determined to be .75. Since significant control is not required at the destination, then the RWL is onIy computed at the origin of the lift. The multipliers are computed from the lifting equation or determined from the multiplier tables (Tables 1 to 5, and Table 7). As shown in Figure 19, the RWL for this 1ifting task is 16.4 Ibs.

3.3.3.3. Hazanl Assessment

The \\cight to be lifted (25 Ibs) is greater than the RWL (16.4 Ibs). Therefore, the Il is 25/16.4 or 1.5. This task would be stressful for some bealthy workers.

79

30 INCHES

Figure 18 Packaging I, Example 6

80

00 .....

JOB ANALYSIS WORKSHEET DEPARTMENT Dislribution .aOB DE8CRIPTION

JOB TITLE Line Packer Packing: Ilroducls for dislribulion ANALY8T'8 NAME DATE Example 6, ProducI Packaging I

STEP 1. MeBaure and record taak varlablea

Object Hand Localion (In) Verticel Asymmetric Angle (degrees Frequency Rate Duration Object Weighl (Ibs) Drigin Dest Oistance (in) Drigin DesUnation lil1s/min (HRS) Coupling

L (AVG.) L (Max,) H V H V D A A F C

25 I 25 14 124 161 40 16 90 O I 8 Fair

STEP 2. Determlne the multlpllera and compute the RWL'a

ORIGIN

RWL = LC • HM • VM • DM. AM. FM. CM I RWL =[ill. WJ·~· ~. [li]. [ill. [M] = 16.4 Lb. I

DESTINATION RWL =[ill·D·D·D·D·D·D= I Lb. I

STEP 3. Compute the LIFTING INDEX OBJECT WEIGHT (L) 25 ~ ORIGIN LIFTING INDEX - ~ = 1.5 RWL

DESTINATION LIFTING INDEX -OBJECT WE1GHT (L)

=-=D RWL

Figure 19: Example 6, Job Analysls Worksheet

3.3.3.4. Redesign SuggestiOIlS

The \\Ulksheet shows tbat the nrultiplic:rs with the smaIlest magnitude (i.e., tOOse puviding the greatest penalties) are .71 for the HM, .71 for the AM, and .75 for the fM Using Table 8, the following job modifications are suggested:

i. Bring the load c10ser to the \Wrlrer to increase HM

2. Mwe the lift's origin and destination c10ser toget:her to reduce the angIe oftwist and increase the AM

3. Reduce the lifting frequency rate andIor provide longer recovery periods to increase fM

Assuming tbat the Iarge horizontaI distance is due to the size of the object lifted ratber than the existence of a barrier, then the horizontal distance couId onIy be reduced by making the object smaIler or re-orienting the object. An alternate approach \\OOI.d be to eliminate body twist by puviding a curved chute to bring the object in front of the \\Ulirer. For this modified job (\\UIksheet shown in Figure 20), the AM is increased from 0.71 to LO, the HM is increased from 0.71 to o.n, the RWL is increased from 16.4 Ibs to 25 lbs, and the U is decreased from 1.5 to 1.00. Eiminating body twist reduces the physical stress to an acceptable level for IDJSt workers. Alternate redesign recomIICldations could include: (1) raising the height of conveyor A andIor reducing the height of \\Ofk bench B; or, (2) Providing good coupIing; on the containers. For example, the curved chute couId aIso be designed to bring the load to a height of 30 in:hes. Ibis \\OOI.d increase the VM, DM, and CM vaIues to LO, \Wich \\OOld reduce the lifting index even further.

82

JOB ANALYSIS WORKSHEET DEPARTMENT Distribution "OB DE8CRIPTION J08 TITLE Line Packer Packing Ilroducts for distribution AMALvaT'. HAME DATE Modified Exarnllie 6

STEP 1. Meaaure and record taak varlablea

Objecl Hand location (in) Verticel Asymmetric Angle (degrees Frequency Rate Duratlon Ob)ect Weighl Obs) Origln DasI. Distance (in) Origin DeslinatlOn lifts/min (HAS) Coupling

L (AVG.) I L (Max.) H V HT V D A A F C

25 1 25 13T24 16140 16 O O I 8 Fair

es STEP 2. Oetermlne tha multlpllera and oompute the RWL'a RWL = LC • HM • VM. DM. AM. FM. CM

ORIGIN RWL =[!IHllJ· [~iHill· [ll]. [li] . [M] = I 25.0 Lbo I DESTINATION RWL =[ill·D·D·D·D·D·D= I Lbo I

STEP 3. Compute the LIFTING INOEX OBJECT WEIGHT (L) 25 [;] ORIGIN LIFTING INDEX - = -zsF = 1.0 RWL

DESTINATION LIFTING INDEX -OBJECT WEIGHT (L)

--=D RWL

Figure 20: Example 6, Modlfled Job Analysls Worksheet

3.3.3.5. ComllKmls:

Although severa! alternate redesign suggestiom are provided, reducing 1he asytmlletric angle should be given a high priority because a significant number of overexertionlifting injuries are associateci with excessive lumbar rotation and flexion

As in 1he earlier examples, 1he lifting pattem is continUOIl'l over 1he full dmation of 1he \\UI'k sessions. Thus, 1he lifting fu:quency is not adjusted Il'ling 1he specia1 procedure described in 1he Frequency Component section OlI page 27.

3.4. Repetitive Multi-Task, Sbort-Dumtion

3.4.1. Depalletizing ~r.dion, ~e 7

3.4.1.1 • .lIb Description

A \\UI'ker tmloads 12-1b cartons from a pallet onto a conveyor, as illustrated in Figure 21. The cartons are vertically stacked fiom 1he floor in five tiers. No twisting is required \\hen picking up and putting down 1he cartons, and 1he \\Ufker is free to step OlI 1he pallet to get dose to each carton (i.e., only one layer in depth from 1he front of 1he pallet nrust be analyz.ed). WaIking and carrying are minimized by keeping 1he pallels dose to 1he conveyor, and significant control of 1he object is not required at 1he destination of 1he lift. The vertical location (V) at 1he origin, horizontallocation (H), and vertical trave1 distance (O), vary fiom one lift to 1he next.

3.4.1.2. .lIb AnaIysis

Since 1he job comists of more than one distinct task and 1he task variables often change, 1he multi-task lifting analysis procedure should be used.

84

fii Ili fii

·flJ~ i'; ra

.. I 421

t-

i §

i ! .. .. :1

! .... M

I

85

This job is divided into five tasks iepiCscllting the five tiers of loaded paIIets. Task numbering is atbitraIy and the sequencing does not reflect the 0Ider in which the tasks are perform::d. It is in:qxn: tant, however, to identifY each distinct type of lifting task. Note, it may not be apptOpiiate to use the lifting equation for mixed-task jobs 1hat require significant anx>unts of pushin& pulling, or canying.

The following meast.n"CilCItsobservatiom were made and rccorded on the job anaIysis \\Ul"ksheet (Figure 22):

l. Carton dimensiom are 16 inches x 16 inches x 16 inches.

2. The vertica1locatiom at the origin represcnt the position of the hands under the carton'I. The top of the conveyor is 20 inches from the floor.

3. For this example, assume 1hat the horizontallocatiom were not nrasured, but estimated lEing the formuIas provided in the HorizontaI Multiplier section on page 14. From thesc formulas, H = (8 + 1612) or 16 inches for the top four tiers and H = (lO + 1612) or 18 inches for the bottom tier.

4. The paIIet is 4 inches in height.

5. No asylmnetric lifting is involved (i.e., A = O).

6. Cartons are continuously lDlIoaded at the tate of 12-per minute (Le, 2.4 liftsImin per tier) for l hour.

7. The job comists of continuous l-hour \Wik sessiom separated by 9O-minute rccovery pcriods.

8. Using Table 6, the conpling is cIassified as fair.

86

DEPARTMENT R~e~Cl~·:.::e:.:.Vl::;·n:l;g!.-___ _ .JOII TITLE Warehouseman ANA~sr8NAME ________________ _

2 3

~

Figure 22: Example 7, JOB ANALYSIS WORKSHEET

The multi-task lifting anaIysis consists of tbe following three steps:

l. Compute tbe trequm;y-iIKlependent-RWL (F1RWL) and trequm;y-iIKlependent- lifting index (Fili) values for each task ming a default FM of 1.0.

2. Compute tbe single-task-RWL (S1RWL) and single-task-lifting index (SITl) for each task Note, in this example, intetpo1ation was used to compute tbe FM value for each task becaIl'le tbe lifting trequm;y rate was not a \\bele number (i.e., 2.4).

3. Renumber tbe tasks in order of decreasing physica1 stress, as determined from tbe STIl value, starting with tbe task with tbe largest STIl.

Stetù Compute tbe FIRWL and Flll values for each task ming a default FM of 1.0. The multi-task lifting anaIysis consists of tbe following three steps:

l. Compute tbe trequm;y-iIKlependent-RWL (F1RWL) and trequm;y-independent- lifting index (Fili) values for each task ming a default FM of 1.0.

FIRWL FILI Tier 1 20.4 Ibs .6 Tier 2 28.4 Ibs .4 Tier 3 28.71bs .4 Tier 4 23.8 Ibs .5 Tier 5 19.9 Ibs .6

These resu1ts indicate that none of tbe tasks are particu1arly stressful, from a strength point of view, but that tiers l and 5 do require tbe most strength. Remember, however, that these resu1ts do not take tbe trequm;y of lifting into consideration.

88

Step..2. Compute the S1RWL and SIll values for each task, where STRW... = FIRIM.... x FM. The 1M for each task is determined by inteIpoIating berneen the 1M values for 2 and 3 lifts/minute from CollllIBl 2 of Table 5. The results are displayed in Figure 22.

STRWL STLI Tier 1 18.4 Ibs .7 Tier 2 25.6 Ibs .5 Tier 3 25.8 Ibs .5 Tier4 21.41bs .6 Tier 5 17.91bs .7

These results suggest tha none of the tasks ere stressful, if peiformed individudly. Note, however, tha these vdues do not consider the combined effects of dI of the tasks.

Renumber the tasks, starting with the task with the largest SIll value, and ending with the task with the sma1lest SIll value. If more than one task has the same 8111 value, assign the lower task nwnber to the task with the highest frequency.

3.4.1.3. Ha:nud Assessmellt

Compute the composite-lifting index (CLI) for the job, using the renumbered tasks as described in the Multi-Task procedures on page43.

As shown on Figure 22, the CI1 value for this job is 1.4. This means that some healthy \\Qrkers would find this job physically stressful. Therefore, some rerlesign may be needed Analysis of the results suggest that any three of these tasks would probably result in a CI1 below LO, which would be acceptable for nearly ali healthy \\Qrkers. However, when the other two tasks are added, the overall frequency increases the lifting index above 1.0. This suggests that the overall frequency should be reduced to limit the physical stress associateci with this job.

89

3.4.1.4. Redesign SuggestiOIl'i

1be worksheet illmtrated in Figure 22 indicates that the nruItipliers with the smallest magnitude (i.e., those puviding the greatest penalties) are .56 for the HM at TIer l; .63 for the HM at Tiers 2 through 5; .72 for the VM at TIer 5; and .81 for the VM at TIer l. Using Table 8, the following job mxlliications are suggested:

1. Bring the cartom closer to the worker to increase the HM vaIue.

2 Lmwr the beight for TIer five to increase the VM vaIue.

3. Raise the beight of tier ore to increase the VM vaIue.

1be FlLI vaIues are aIlless than LO, indicating that strength sbouJ.d not be a problem for any of these tasks. Mlreover, the S1U were aIlless than LO, indicating that none of the tasks VIOOld be physica1ly s1ressful, if performed individually. Wben the combined physica1 demands of the tasks are COIl'Iidered, bowever, the resulting al exceeds 1.0. 1bis is likely due to the high frequency rate for the combined job. Since a mnnber of simplifying asswnptions were made in this example, however, a IOOre detailed Iretabolic analysis of such a job may be needed before implem::nting ergonomic redesign. Such an analysis is described in detail by Garg et d. (1978).

An engineering approach sbouJ.d be the first cboice for job redesign (i.e., physica1 changes in layoot, such as raising or lmwring shelves, tables, or pallets) rather than worker compliance. In this case, the high frequency rate is a significant problem and sbouJ.d be red"m A redt.x:tion in frequency cou1d decrease the al to about 1.0.

90

3.4.1.5. Conmenls

With roore complicated tasks, such a simple solution wi11 not necessarily be possible, and roore detailed ana1yses may be required to detennine compiessive forces, strength requirements, and energy expenditures.

3.4.2. Harvling Gms or licpd, ~e 8

3.4.2.1 • .lIb Description

A \Wrlrer unloads cans of Iiquids from a cart to three storage shelves as sho\W in Figure 23. Although the cans are Iifted in the sagittal piane vffien. moved between shelves, they are usua11y Iifted asynnnetricaIly, from one side of the body to the other, vffien. Iifted from the cart to the shelves. The worker may take a step vffien. p1acing the cans onto the shelf. The cans do not have nx.Jlded handholds, so the \Wrlrer hooks his fingers or slides his hand under the tmned edge of the can to Iift it When Iifting to the top shelf, mxkers usua11y reposition their grip near the end of the Iift. The mxk pattem consists of intennittent, six-minute mxk sessions separated by three-rninute recovery periods. The actua1 Iifting frequency during the six-rninute mxk sessions was 9 Iifts/rninute. There is a 9O-rninute txeak after each hour of work.

3.4.2.2. .lIb Analysis

Since the job consists of roore than one distinct task and the task variables change often, the multi-task lifting ana1ysis procedure shou1d be used.

91

~

ASYMMETRY LINE AT ORIOIN

Figure 23 Handling Cans of Liquid, Example 8

This job is divided into three tasks. Task 1 is defined as lifting from the aut to the 10\Wf shelf. Task 2 is defined as lifting to the center shelf, and Task 3 is defined as lifting to the upper shelf. Since task 3 requires a reposition of grip at the destination, it must be analyzed at both the origin (fask 3a) and the destination of the lift (fask 3b). The left and right shelf positions are considered to be equivalent, since the 'MJfker can step toward the shelf during the lift.

The following task variable data were meastn"ed and recorded on the job analysis 'MJfksheet (Figure 24):

l. Cans are 8 inches in height.

2. Cart is 15 inches high.

3. Shelf 1 is 2 inches high.

4. Shelf 2 is 22 inches high.

5. Shelf 3 is 42 inches high.

6. At the origin, the horizontal distance (II) is 17 inches, the vertica1 height (V) is 23 inches, and the angle of asymmetry (A) is 4SO for alllifts.

7. At the destination, H is 22 inches, and A is 4SO for alllifts.

8. The cans are lifted in an intennittent 'MJfk pattem at a rate of 9 lifts/min (i.e., 3 lifts/min per shelf) for a duration of 1 hour.

9. Using Table 6, the couplings are c1assified as poor.

The nru1ti-task lifting analysis consists of the following three steps:

l. Compute the frequency-independent-RWL (FlRWL) and frequency-independent - lifting index (FIIl) values for each task using a default 1M of 1.0.

93

2. Compute the single-task-RWL (S1RWL) and single-task-lifting index (S1LI) for each task. Note: Since the \\Urk prttern is not continoous for the 15-minute sample, the lifting frequency is adjusted using the special procWure described on ptge 27.

3. Renumber the tasks in arder of decreasing physical stress, as determined from the STIl value, starting with the task with the largest STIl.

Stetù

Compute the F1RWL and Flll values for each task using a default 1M of 1.0. The other nrultipliers are computed from the lifting equation or determined from the nrultiplier tables (fable 1 to 5, and Table 7). The F1RWL and Flll values are computed only at the origin fOl' Tasks 1 and 2, but since significant control is required for Task 3, the values must be computed at both the origin and destination of the lift.

Task 1 Task2 Task 3a Task 3b

FIRWL FILI 21.2 Ibs 1.4 22.11bs 1.4 19.7 Ibs 1.5 13.7 Ibs 2.2

94

These results indicate that aIl of the tasks may req,.re consideroble strength, especially al the destination of Task 3. Remember, however, that these results do not take the frequency oflifting into consideration.

Compute the SIRWL and STIl values for each task, \\bere the SIRWL for a task is equivalent to the product of the FIRWL and the FM for that task. In this example, the \\\JIk pattem is intermittent so the frequency is adjusted using the special procedure. Thus, for this job, F = (3 lifts/minute x 6 minuteslperiod x 2 periods) / 15 minutes, \\bich is equa! to 36115, or 2.4 lifts/minute. As in the previous example, the FM values must be detennined by interpolating between the FM values for 2 and 3 lifts/minute from Column 2 ofTable 5. The results are displayed in Figure 24 and summarized below.

Task 1 Task 2 Task 3a Task 3b

STRWI.. STLI 19.1 Ibs 1.6 19.9 Ibs 1.5 17.71bs 1.7 12.4 Ibs 2.4

These results indicae tha dI 01 the tafks would be paticulaly stressjùl, if peiformed individudly. Note, however, tha these vdues do not consider the combined effects 01 dI 01 the tafks.

Renumber the tasks, starting with the task with the largest STIl value, and ending with the task with the smallest STIl value. If more than one task has the same STIl value, assign the lower task mnnber to the task with the highest frequency.

95

~

MULTI.TASK JOB

D!PARTM!NT :::Pai=' n~t-:S:::h=:oPI7-___ _ JOII T1TU! Stock Clerk ANALYBrBNAM! ________________ __

DAY!

STEP 1.

TtI.k No.

2 '3

•• nd ì .. k ObJeel

~.I"rtb.

Y . L ax.

3D 30

30 I 30 1171 231 30 30 17 23

I I I I ! I I

.. JOII DEBCRIPTION

Lifting cans of liquid from from cart to shelves

18 De ..

Il /"srÒiii' • (In) gn ~ I CO~llng

<llPoor ~l pt")or

STEP 2. Computa multlplI.r. end FIRWL, STRWL, FILI, end STU 'or Eeeh T .. k

~,,:k LC. HM • VM • DM • AM • CM ~IRWL. FM 8TRWL U~~WL JWRWL T':::~O. F

I Bl .59 .95 .96 .86 .90 21.2 .90 19.1 1.4 1.6 2 2.4

2 BI .59 .95 1.0 .86 .90 22.1 .90 19.9 1.4 1.5 3 2.4

38 BI .59 .95 .89 .86 .90 19.7 .90 17.7 1.5 1.7 >< 2.4

3b BI .46 .85 .89 .86 .90 13.7 .90 12.4 2.2 2.4 I 2.4 BI

! , , I r STEP 3. Computa th. Uftlng ."de. for the Job (Afte, renumberlng tnk.: A FILI, + 2S FILI, + A FILI, I .MIoIo•• 1/'11 .. ,,1 'IL .. '1' ............ 1 ---

CLI ., BTU, + A FILI. + l'ILI,C111M .... 1"",)

.4(\/.7 . 1/.8) 1.40/.8 . 1/.9) ICLI.{ 2.4 .19 :25 r2F

Figure 24: Example 8, JOB ANALYSIS WORKSHEET

3.4.2.3. Hazanl AssessmeDt

Cornpute the composite-1ifting index (CU) using the renumbered tasks. Recall that a special procedtn'e is used to detennine the appropriate FM values \\ben (I) repetitive 1ifting is perfonned for short duratiOIl'l, and (2) sufficient recovery periods are provided. For example, the frequency for each task in this example is detennined by multiplying the actual frequency rate (3 lifts per minute) times the duration (12 minutes), and dividing the result by 15 minutes to obtain an adjusted frequency rate of 2.4 lifts per minute, which is used to compute the CLI.

As shown in Figure 24, the CIl for this job is 2.9, which indicates that there is a significant level of physica1 stress associated with this job. It appears that strength is a problem for ali three tasks, since the FIIl values ali exceed 1.0. Therefore, the overa11 physica1 demands of the job are primarily the result of excessive strength demands, rather than the lifting frequency rate. This may not be the case ifthe duration exceeds 15 minutes, due to an increase in eIXiurance demands.

3.4.2.4. Redesign Suggestiom

The \\\JI'ksheet illustrated in Figure 24 sho\W that the multipliers with the smallest magnitude (i.e., those providing the greatest pena1ties) are .46 for the HM for Task 3 at the destination; .59 for the HM for Tasks I, 2, and 3 at the origin; .85 for the VM for Task 3 at the destination; .86 for the AM for alI tasks at the origin and destination; and, .90 for the CM for alI tasks.

Using Table 8, the following job modifications are suggested:

1. Bring the load closer to the \\\JI'ker to increase HM by reducing the size of the can andIor brioging the load betwcen the \\\JI'ker's legs.

2. Reduce the angle of twist to increase AM by moving the origin and destination closer together or fi.nther apart.

97

3. Provide contaiIx:rs with handles or bandhold cutouts to increaseCM

4. Raise the origin of the lift to increase VM

Raising the vertical height al the origin \\OOld also decrease the vertical dispiace" ent (D), and redoce the angle of twist. Since the size of the H value al the origin depems on the size of the container, the only way to redoce H \\OOld be to redoce the container size. An additional berrlt of reducing container size is an accompmying reduction in H al the destination for Task 3.

If(l) the height ofthe cart is in:reased, (2) twisting is eliminaterl, and (3) Task 3 is deleted, then the FIRWL for Tasks 1 and 2 \\OOld be 27.1 lbi (i.e., 51 x .59 x 1.0 x 1.0 x 1.0 x 1.0 x 0.90), and the F111 \\OOld be redllced from 1.4 to 1.1, \\bich \\OOld be acceptable to many JOOre \\QI'kers than before.

As an alternative, an engineering modification could include a design that aIlows the sbelves to either revolve vertically or rotate horizontaIly for JOOre storage space al the optimum lifting height of 30 iIrhes. 1bis design \\OOld eliminate the n:ed to bend or reach \\bile lifting, \\bich is a safer design.

3.4.2.5. OImmenIs

In this example, the cans \Vere not stacked bigher than a single can on the cart. The cans, however, could be stacked bigher. For a second Iayer, the vertical height (V) al the origin \\OOld be near knuclde height (Le., about 31 iIrhes). The vertical nrultiplier (VM) \\OOld be increased and the FIRWL \\OOld be bigher than for lifting from the I~ Iayer, thus reducing the risk. A third Iayer, however, may increase the risk of overexertion uyury and result in a JOOre stressfuI job for som: \\QI'kers.

98

3.5. Repetittve Multi-TIl'ik, I.ong-Dumtion (> 2 brs)

3.5.1. ProWct Pacliaging IJ, ~e 9

3.5.L1 • .IIb Drscription

RoIIs of paper \\cighing 25 Ibs each are puIIed off a IJX)ving conveyor to work stations where they are WIapp::d and placed in boxes, as shown in Figure 25. Conveyor delivery aIlow.; the roll to slide to the wrapping area, but the roll must be manipulated as it is wrapped After wrapping, the roll is Iifted from the table and placed in a box. The box is cIosed, secured, and Iifted to a paIIet. The \Wrlrer completes this operation once per miIrute for a continuous duration of 8 hours. The \Wrlrer does not twist when lifting the roIIs of papero The first Iift (from the table to the box) requires significant control at the destinatiOIL The second Iift (from box to paIIet) does not require significant control at the destination

3.5.1.2. .IIb Analysis

Since the job consists of more than one task, the nrulti-task lifting anaIysis procedure shouId be used. Task l consists of lifting the roll of paper from the table and pIacing it into a cardboard box, and Task 2 consists of lifting the loaded box from the floor onto the paIIet. No 3S)'nHnetric lifting is involved in either task (i.e., A = O). The following task variable data \Wre measured and recorded on the job anaIysis worksheet (Figure 26).

rask ).

1. At the origin of the Iift, the horizontaI distance (lI) is 21 inches and the verticaI distance (V) is 38 inches.

99

100

-- ----

MULTI-TASK JOBANALYSIS WORKSHEET

DEPARTMENT ShiEEing JOB DESCRIPTION

JOB TITLE Packager Wrapping and boxing products ANALYST'S NAME and lifting them to a Qallet DATE ExamQle 9, Product Packaging II STEP 1. M ••• ur. and Record T •• k Varlable Data

ObJect Hard LoCatlon (In) Vertlcal As mmet le (d $) Fra ne Rate Duration Coupllng T .. k No. w., hl (Ibs) ngln ." Istanc:a (ln) ng'n asI. Ilfts/mln H .. L , L Max_) H V H V D A A F C

I ~R 25 2 .38 r IO 36 2 O R -PnnT

2 ?il 25 IO O IO 6 6 O O I 8 Fair

-o - STEP 2. Computa multlpllare end FIRWL, STRWL, FILI, end STLI lor Eeeh T.ek T •• k LCx HM x VM x DM x AM x CM FIRWL x FM STRWL ~~RWL ST~L- N.w F No. L/STRWl T •• k No.

la 51 .48 .94 1.0 1.0 .90 20.7 .75 15.5 1.2 1.6 I I

Ib 51 1.0 .96 1.0 1.0 .90 44.1 .75 33.1 .6 .8 >< I

2 51 1.0 .78 1.0 1.0 .95 37.8 .75 28.4 .7 .9 2 I 51

51

STEP 3. ComDute the Composite Ultlna-Index lor the Job IAII., ronumborlng t.aka) CU· STll i + ~ FILI, + ~FILI. + ~ FILI, + ~ FILI,

'ILI.(1/f'M, .... 1/''',) FILI,(1/FM ••• +,· 1/1"" ... ' flLI.(1/F .. " ...... 11P ...... JFI1oI.f1I"M • 1/1""

. 7111.65-1/. 751 I CU 1.6 .14 1.7

FIgure 26: Example 9, JOB ANALYSIS WORKSHEET

2. At the destination of the Iift, H is lO inches and V is 36 inches.

3. If the roIIs are handled lengthwise, as shown in Figure 25, then the couplings are classified as "poor", because the fingers can't be flexed near 9fJ1. (See Table 6).

Task2:

l. At the origin of the Iift, H is lO inches and V is O inches.

2. At the destination ofthe Iift, H is lO inches and V is 6 inches.

3. 1he couplings are classified as "fair" because the fingers can be flexed under the box about 9fJ1 (See Table 6).

1he lifting frequency rate for each task is l liftlminute. This m:ans that nw Iifts occur each minute, since both Task l and Task 2 occur about once per minute.

1he nrulti-task lifting analysis consists of the following three steps:

l. Compute the frequency-independent-RWL (FIRWL) and frequency-independent- lifting index (FITJ) values for each task ming a default FM of 1.0.

2. Compute the single-task-RWL (SIRWL) and single-task-lifting index (S1LI) for each task.

3. Renwnber the tasks in increasing order of physical stress, as determined from the S1U value, starting with the task with the Iargest S1U.

~

Compute the F1RWL and Flll values for each task ming a default FM of 1.0. 1he other nrultipliers are computed from the lifting equation or determined from the nrultiplier tables (fable l 10 5,

102

and Table 7). Since Task l requires significant control at the destination, the FIRWL vaIue lIlUSt be caIculated at both the origin (Task la) and the destinatiOll (Task lb) of the lift.

Task la Task lb Task2

FIRWL 20.71bs 44.1 Ibs 37.81bs

FILI 1.2 .6 .7

The resu1ts indicate that these tasks should not reqlire excessive strength. Remember, lxMever, that these resu1ts do not take the frequency of lifting into consideration

Compute the S1RWL and STI1 vaIues for each task, \\here the S1RWL for a task is equivaIent to the product of the FIRWL and the FM for that task. Based 011 the given frequencies, verticaI heights, and durations, the FM vaIues are detennined frmn Table 5.

The resu1ts are displayed in Figme 26 and S\IIIlIIl3rized below.

Task la Task lb Task2

STRWL 15.5 Ibs 33.1 Ibs 28.4 Ibs

STLI 1.6 .8 .9

These resu1ts indicate that, if perfonned individually, Task 2 'Mluld not be stressful, but that Task l woWl be strr!SSf1i for some healthy 'MlIkers. Note, ho\WVeT, that these vaIues do not consider the combined effects of all of the tasks.

Renumber the tasks, starting with the task with the largest STI1 vaIue, and ending with the task with the smaIlest sru vaIue. If more 1han one task has the same STI1 vaIue, assign the lower task nwnber to the task with the highest frequency.

103

3.5.1.3. lIa:TJud Assessment

Compute the collljXJSite-lifting index (CIl) using the renumbered tasks. OnIy the origin or destination component with the Iargest S1ll is used to compute the ili for the job when significant control is required for a task. As shown in Figure 26, the ili for this job is 1.7, \Wi.ch indicates that this job ",olitI be physicdly stressfd for some heiithy ",oTkers.

3.5.1.4. Redesign SuggestiOIl'l

The \Wlksheet illustrated in Figure 26 shows that the multipliers with the smallest magnitude (Le., those puviding the greatest penalties) for this task are .48 for the HM at the origin ofTask l, .78 for the VM for Task 2, and .90 for the CM at the origin and destination of Task 1. Using Table 8, the following job IOOdificatiOll'l are suggested:

1. Bring the load doser to the \WI'ker to increase HM by reducing the size of the roll andIor l:ringing the load between the \Wrlrer's legs at the origin for Task 1.

2. Raise the vertical height of the lift for Task 2 at the origin and at the destination to increase VM

3. Provide better couplings for Task l to increase CM

The Iargest penalty com:s ftom lifting the rolls ftom the wrapping table into the box. A prnctical job redesign \\Ould be to provide a recess for the box at the end of the table, so that the \WI'ker couId easily slide the roll into the box without lifting it The \\QI'ker couId then slide the box to the edge of the table, and lift it ftom the table to the pallet This job IOOdification \\Ould aIlow the \WI'ker to get doser to the load when lifting, \Wi.ch \\Ould increase the FIRWL and decrease the FILI.

104

As an alternative job modification, the 'M>rker could be rotated from this job to a job with light 'M>rk every one to two hours to decrease the lifting duration. This 'M>uld provide a sufficient recovery period for the 'M>rker, so that fatigue 'M>uld not become a problem. The light duty 'M>I'k, ho~er, should 1ast for at least .3 times the arnount of time spent on the packaging job.

3.5.1.5. CoIIilitilt\

There is an inherent danger in trying to simplify a complex lifting job. The overriding concern is that the worker is not exposed to excessive biomechanicaJ or physiologicaJ stress. This multi-task analysis procedure was designed to provide a series of intermediate values that 'M>uld help guide the redesign of physicaJly demanding lifting tasks. These values include the FIRWL, FIIl, S1RWL, and STIl. These intennediate values should not be used as design limits, since they on1y provide task specific infonnation. The 0vera11 risk of injury for a lifting job is dependent upon the combined effects of the job, rather tban the individuai effects of the tasks.

3.5.2. Warehome OnJer Filling, Exaiqje lO

3.5.2.1 .bb Description

A 'M>rker lifts cartons of various sizes from supply shelves onto a cart as illustrated in Figure 27. There are three box sizes (i.e., A, B, and C) of various weights. These 1ifting tasks are typicaJ in warehousing, shipping, and receiving activities in wruch loads of varying weights and sizes are lifted at different frequencies. Assume that the following observations were made: (1) control of the load is not required at the destination of any lift; (2) the 'M>rker does not twist when picking up and putting down the cartons; (3) the worker can get dose to each carton; and, (4) wa1king and carrying are minimized by keeping the cart dose to the shelves.

105

IB

2 BI ;

o ..... II)

l 011 .5 -re ~ ~ J t--N

~ li:

106

3.5.2.2. .bb Analysis

Since the job consists or more than one distinct task and the task variables often change, the IIRÙti-task lifting analysis procedtn"e should be used.

This job can be divided into three tasks represented by cartons A, B, and C. The fo11owing measurements were made and recorded on the job analysis worksbeet (Figure 28):

L The horimntallocations (lI) for each task at the origin and destination are as fo11ows: Box A, 16 inches; Box B, 12 inches; and, Box C, 8 inches.

2. The vertica1locations (V) at the origin are taken to be the position of the hands under the cartons as follows: Box A, O inches; Box B, O inches; and, Box C, 30 inches.

3. The vertica1 locations (V) at the destination are the vertica1 position on the cart as fo11ows: Box A, 30 inches; Box B, 6 inches; and, Box C, 39 inches.

4. The average weights lifted for each task are as fo11ows: Box A, 22 100; Box B, 33 100; and, Box C, Il 100.

5. The maximwn weights lifted for each task are as fo11ows: Box A, 33 100; Box B, 44 100; and, Box C, 22 100.

6. No asylmnetric lifting is involved (Le., A = O).

7. The lifting frequency rates for each task are as fo11ows: Box A, 1 liftIrnin; Box B, 2 lifts/rnin; and Box C, 5 lifts/min

8. The lifting duration for the job is 8 homs, however, the maximwn weights are lifted infrequently (Le., less than or equa! to once every 5 minutes for 8 hours)

9. Using Table 6, the coupling-; are classified as fair.

107

MULTI·TASK JOB ANALYSIS WORKSHEET

D!PARTMENT Warehouse JOI D!8CRIPTION

JOI TlTL! ShiEEing Clerk Selecting an order for shiEment ANALY8T'8 NAMI Warehouse order filling DATE ExamEle lO STEP 1. M ••• ur. and Record T •• k Varl.bl. O •••

T •• k No. Objeci Hand Location (In) v.mc., A. mmat Mg1e (da l) F,.. A.t DUlallon Coup!!ng Il ;'~""-hI Ibl\ gn e.1. Iltane. (In) , n •• IIIII/min H •

L ~ H V H V O A A , C

I(Al 33 16 O 6 30 O l 8 Falr 2 (ll) 33 44 110 O 110 6 6 O 2 8 Fair 3 (Cl 11 22 8 130 8 39 Q O B 8 F.ir

-156 STEP 2. Comput. multlpll •• a and FIRWL, STRWL, FILI, and STU lor E.eh T.ak

~!k Le x HM x VM x DM x AM x CM FlRWL • ~M STRWL L~~:-'R;L Il~~~~WL ,New T.n No. F

l al .63 .78 .88 1.0 .95 21.0 .75 15.8 1.6 1.4 2 l

2 al .83 .78 1.0 1.0 .95 31.4 .65 20.4 1.4 1.6 l 2

3 al 1.0 1.0 1.0 1.0 1.0 51.0 .35 17.8 .4 .6 3 5 51

al 8TEP 3. ComDut. th. ComDo.lt. Llltlna Ind.x ID' th. Job (Afte ... numbe.lna t .. ko) CU· 8TU t + AFILli + A FlU, + A FILI, + AFILli

'ILI.H/flM .... '''M,' Il'U,CHII ........ "I"MIo., 'ILI.('/PMIo ••••• • "P ....... IIIL1 '''M .1/'" li em:BB-I/.65 .4 W8- 55

CU. 1.6 .45 1.5 3.6

Figura 28: Exampla 10, JOB ANALYSIS WORKSHEET

The muIti-task lifting ana1ysis consists of the following three steps:

1. Compute the frequency-independent-RWL (FIRWL) and frequency-independent-1ifting index (Flll) values for each task using a default FM of 1.0.

2. Compute the single-task-RWL (S1RWL) and single-task-1ifting index (S111) for each task.

3. Renumber the tasks in order of decreasing physical stress, as determired from the SlIl value, starting with the task with the Iargest SlIl.

Step..l

Compute the FIRWL and Flll values for each task using a default FM of 1.0. The other nruItipliers are computed from the lifting equation or determired from the nruItiplier tables (Table l to 5, and Table 7). Recall that the Flll is computed for each task by dividing the maximum weight of that task by its FIRWL.

Task 1 Task 2 Task 3

FIRWL FILI 21.0 Ibs 1.6 31.4 Ibs 1.4 51.0 Ibs .4

These resuJts indicae tha two 01 the tmb require strength demmds tha exceed the RWL level. Remember, however, tha these results do not tdee the frequency 01 lifting into conside1ttion

109

Compute the S1RWL and SILI vaIues for each task, \Were the S1RWL for a task is equivaIent to the product of the FIRWL and tbe FM for that task. Recall that the SI1LI is computed for each task by dividing the average weight ofthat task by its S1RWL. The app:optiate FM vaIues are detennined fiom Table 5.

Task 1 Task 2 Task 3

STRWI.. STLI 15.81bs 1.4 20.4 Ibs 1.6 17.8 Ibs .6

These resulfs indicae tha Tasks 1 ad 2 wauld be stressful lor some w~. if performed individud/y. Note. however, tha tllese vdues do noi consider fhe combined effects 01 di of the fasks.

Renumber the tasks, starting with the task with the Iargest SILI vaIue, and erxIing with the task with the smaIlest SILI vaIue. If more than one task has the sarre SILI vaIue, assign the 1O\\tt task I1UIIlIx:r to the task with the highest fu:queIK:y.

3.5.2.3. Hrnnd Assessment

Compute the composite-Iifting index (CLI) using the renumbered tasks. As sImw in Figure 28, the ru for this job is 3.6, \Wich indicates that this job \\OOId be physicaIly s1ressfuI for nearly ali \Wrlrers. Analysis of the results suggests that the combined effects of the tasks are significantly more s1ressfuI than any individuai task.

110

3.5.2.4. Redesign Suggestiom

Developing a redesign strategy for a job depends on tangible and intangible factors that may be difficu1t to evaluate, including costsIbenefits, feasibility, and practica1ity. No preferred procedure has been developed and tested. Therefore, the following suggestions represent on1y one approach to ergonomic job modification.

In this example, the magnitude of the Fill, SIT1, and al values indicate that both strength and endurance \\U!Ùd be a problem for many workers. Therefore, the redesign should attempt to decrease the physical demands by m:xlifying the job layout and decrease the physiological demands by reducing the frequency rate or duration of continuous lifting. If the maximum weights \Wre e1iminated from the job, then the al \\U!Ùd be significantly reduced, the job \\U!Ùd be less stressful, and more \\mkers could perfonn the job than before.

Those lifts with strength problerns should be evaluated for specific engineering changes, such as (1) decreasing carton size or removing baniers to reduce the horizontal distance; (2) raising or Io~g the origin of the lift; (3) reducing the vertical distance of the lift; improving carton couplings, and 4) decreasing the weight to be lifted. The redesign priority for this example is based on identifying interventions that provide the largest increase in the FIRWL for each task (Step 2 on worksheet). For example, the maximum weight lifted for carton A is unacceptable; however, if the carton at the origin \Wre on the upper shelf, then the FIRWL for Task l \\U!Ùd increase from 21.0 Ibs to 27.0 Ibs. The maximum weight lifted stili exceeds the FIRWL, but lifts of average weight are now below the FIRWL. Additiona11y, providing handles, decreasing box size, or reducing the Ioad to be lifted will decrease the stress of manual lifting.

11l

3.S.2.5. Commem

This example deImnstrates the complexity of analyzing nru1ti-task: liftingjobs. &rors resu1ting from averaging. and emJrS intrrxh-ed by ignoring otlx7 factors (e.g., walking, carrying, holding, pu;hing and pulling activities, and environmentaJ. stressors), can on1y be resolved with detailed biOInX:banica1, metabolic, cardiovascu1ar, and psychophysica1 evaluatiom.

Several important applicatiOll principles are illustrated in this example:

1. The horizontal distance (Il) for Task 3 was less tban the 10.0 inches mininnnn. Therefore, H was set equa! to lO inches (i.e., nru1tipliers must be less tban or equa! to 1.0).

2. The vertica1 travel distance (O) in Task 2 was less tban the lO inches mininnnn. Therefore, D was set equa! to lO inches.

112

GLOSSARY

Adion limit (AL) A term from the 1981 WPG that denotes the weight limit that nearly ali workers can perform safely. The term has been replaced in the 1991 equation with the term Reconnnended Weight Limit (seeRWL).

Angle or AsymmetIy (A) The ang1e between the Asymmetric Line and the Sagitta1 Line of the workets Ixxly, as defined by the worker's neutrallxxly position; measure at the origin and destination of lift and use to compute the Asymmetric Mùtiplier (see Asymmetric Line, Asymmetric Mùtiplier, and Neutra11xxly position).

Asymmetric Multipier (AM) A reduction coefficient defined as (1-(.OO32A», has a maximum value of 1.0 when the load is lifted directly in front of the body and decreases linearly as the Angle of Asymrretry (A) increases.

AsymmetIy Une The auxi1iary 1ine that connects the mid-point of the 1ine drawn between the inner ankIe bones and the point projected down to the floor directly below the center of the band grasp8.

Co~ire lifting Index (01) The term that denotes the overaIl lifting index for a tmùti-task manua11ifting job.

Couping OlMification The three-tiered c1assification of the quality of the coupling between the workets hands and the object (either good, fair, or poor); used in the Coupling Mùtiplier (see CM).

Couping Multipier(CM) A reduction coefficient based on the Coupling C1assification and Vertica1 Location of the lift (values found in Table 7).

113

Distmce Varial:ie (D) The vertica1 travel distance of the hands ~ the origin and destination ofthe lift measured in inch:s or centiJreters; used in the Distance Multiplier (see DM).

Distmce Multipier (DM)-A reduction coefficient defined as (.82 + Q.8/I)), for D measured in iD::hes, and (.82 + (4.5/D», for D measured in centiJreters.

DwatiOD or I.Hting The three-tiered classification (either short, mxIerate, or long) of lifting duration specified by the distrihJtion of work-tUre and recovery-tUre (work pattem).

Frequency or lifting (F) The average number of lifts per mimJte aver a 15 minute period; used in the Frequm:y Multiplier (see FM)

Frequency Multipier (FM) A reduction coefficient that depends upon the Frequm:y of Lifting (F), the Vertica1 Location (V) at the origin, and the Duration of Lifting (values found in Table 5).

Frequency-Independent lifting Index (FlIl) A term defined as (LY(HRWL), identifies iOOividual tasks with potential strength problems, values exceeding 1.0 suggest that ergonomic changes may be needed to decrease the strength denmxIs.

Frequency-Independent Reronmended Weight I..imi1s (F1RWL) A value used in a multi-task assessm:nt; product of a1l the reduction coefficients and the LC, holding FM equa! to tmity; reflects the overaJl strength demands for a single repetition of that task; used in Frequm:y-Jndependent Lifting Index (see Flll).

114

Horizon131 Locafion (li) The horizontal distance between the mid-point of the band grasps projected down to the floor and the mid-point of the 1ine between the inner ankIe bones; Il'led in the Horizontal Mùtiplier (see HM).

Horizon131 MultiPier (HM) A reduction coefficient defined as IOIH, for H measured in inches, and 25/H, for H measured in centimeters.

lifting Index (Il) A tenn defined as IJRWL; generally reIates the level of physical stress associated with a particular manual lifting task to the number ofworkers who should be able to perform the task (see Load Weight). A value of 1.0 or more denotes that the task is lmardous for some fraction of the population.

liftingT~k A tenn denoting the act of manually grasping an object of definable size and mass with t\W hands, and vertically moving the object without mechanical assistance.

Load Cons1ant (LC) A constant tenn in the RWL equation defined as a fixed weight of 23 kg or 51 lb; generally considered the maxinrum load nearly alI healthy workers should be able to lift under optimal conditions (i.e. alI the reduction coefficients are unity).

Load Weigbt (L) A tenn defining the weight of the object to be lifted, in pounds or Newtons, including the container; Il'led in the Lifting Index (see li)

l.ong-dmdion A tenn defining lifting tasks that bave a duration of between t\W and eight hours with standard industriai rest aIlowances (e.g., moming, lunch, and aftemoon rest breaks).

115

Mxlelllle-Wration A tenn defining lifting tasks that bave a duration of between one and two hours, follo\Wd by a recovery period of at least 0.3 times the work time [i.e., at least a 0.3 recovery-time to work-time ratio (RTIWI)].

Poor Couping A tenn defining a poor hand-to-object coupling that generally requires bigher maxinrum grasp forces and 1hus specifies a decreased acceptable m:ight for lifting.

RecolJllllended Weigbt limit (RWL) The product of the lifting equation; the load that nearly ali beal1hy wodcers could perfonn over a substantial period of time for a specific se! of task conditiorn.

SagiUalfine The fine passing through the mid-point between the inner anlde bones and lying in the sagittal piane, as defined by the neutral body position

Short-clmltion A tenn defining lifting tasks that bave a work duration of one hour or less, follo\Wd by a recovery time equal to 1.2 times the work time [i.e., at least a 1.2 recovery-time to work-time ratio (RTIWI)].

Significant Control A tenn defining a condition requiring "JreCision placement" of the load at the destination ofthe lift (e.g.: 1. the \\Orlrer has to re-grasp the load near the destination of the lift, 2. the worker has to momentarily hold the object at the destination, or 3. the worker has to position or guide the load at the destination).

Single-Task lifting Index (Slll) A tenn defined as (LY(SIRWL); identifies individuai tasks with potentially excessive physical demands and can prioritize the individuai tasks according to the magnittxle of their physical stress;

116

values exceeding 1.0, suggest that ergonomic changes may be needed to decrease the averaIl physica1 demands of the task.

Singl~ TMk Recommended Weigbt limit (S1RWL) A value used in a multi-task assessrrent; the product ofFIRWL and the awropIiate FM; reflects the overaIl demands of that task, assuming it was the only task being performed. May be used to help determine if an individual task represents excessive physica1 demand; used in Single-Task Lifting Index (see S1LI).

Vertical Location (V) The distaoce of the hands above the floor m:asured at the origin and destination of the lift in ioches or ceotimeters; used in the Vertica1 Multiplier (see VM).

Vertical MultiPier (VM) A reduction coefficient defined as (1-(.0075 IV-30J)), for V measured in inches, and (1-(.003IV-75 I), for V measured in centimeters.

Wdd1("') The width of the container in the sagittal pIane.

117

ASPHINIOSH, 1986, Proposed Naiond Straegies for the Prevention of Lea:ling Work-Relded Disemes ari Iriuries, Pat I, Association or Schools of Public Hea1th llIIder a coopetati ve agleemeut with the Nationa1 Imtitute for (Xcupationa1 Safety and Hea1th, Wasbington D.e

Ayoub, MM and Mital, A 1989, Marud Maerids Ha7dling, (faylor & Francis, London).

0Iaffin, D.B. and Andersson, G.B.1. 1984, Occupaiond Biomechtrtics, (101m Wùey and Sons, New York)

OOL(BLS), 1982, Back ~uries Associated with Lifting, Bulletin No. 2144. U.S. Department ofLabor, Bureau ofLabor Statistics.

Fastman Kodak Company, Ergonomics Group, 1986, Frgrmomic Designfor People a Work, Vol. 2, (Van NostraIxl Reinhold, NewYork)

C .... llagher, S., Marras, W.S., and Bobick T.G. 1988, Lifting in stooped and kneeling postures: effects on lifting capacity, metabolic costs, and elecbonl)'Ography of eight 1nmk =Ies, 1ntemaiond.Joumd qf 1ndustrid Ergonomics, 3: 65-76.

C .... llagher, S. and Unger, RL 1990, Lifting in four restricted lifting con:litions: p;ychophysical, physiologica1 and biomechanica1 effects of lifting in stooped and kneeling postmes, Applied Frgrmomics, 21, 237-245.

C .... llagher, S. 1991, Acceptable weights and physiologica1 costs of performing combined manuaI handling tasks in restricted postmes, Ergonomics, 34(7): 939-952.

118

Garg, A 1991, Fpidemiologicd Ba;is for Mmud Lifting Guidelines, NIOSH Project Report (AvaiJable from the National Teclmical Information Service, NITS munber 91-227-348).

Garg, A, 0Jaffin, D.C., and Herrin, GD. 1978, Prediction of metabolic rates for manual materials handling jobs, A mericm Industrid Hygiene Associt.tion.Joumd, 39(8):661-764.

National Safety Council, 1990, A ccident Fa:ts, National Safety Council, OIicago, ll.

NIOSH 1981, Worlc Pra:tices Guide for Mmud Lifting, NIOSH Teclmical Report No. 81-122, (U.S. DepartnaIt ofHealth and Human Services, National Imtitute for Occupationa1 Safety and Health, Cincinnati, OH).

Waters, T.R 1991, Strategies for assessing muIti-task manual lifting jobs, Proceedings of the Hurrun Fa:tors Society 35th Annud Meeting - 1991, San Francisco, California

Waters, TR, Putz-Anderson, V., Garg, A and Vme, LJ. 1993, Revised NIOSH equation for the design and eva1uation of manuallifting tasks, Ergonomics, 36(7):749-776.

119

APPENDIXI

ERGONOMUCS, 1993, VOL. 36, NO.7, 749-776

Rapid Communication

Revised NIOSH equation for the design and evaluation or manual lifting tasks

THOMAS R. WATERSt, VERN PvTz-ANDERSONt, ARUNGARG'I, and LAWRENCEJ. FlNEt

* Nationa] Institute for Occupational Safety and Health. 4676 Columbia Parkway, Cincinnati, OH 45226. USA 'I Department of IndustriaI and Systerns Engineering, University of Wisconsin-Milwaukee. Wl 53201, USA

Keywords: Low back paio; Prevention and controI; Evaluation methodology; Lifting.

In 1985. tbe National Institute for Occupational Safety and Health (NIOSH) convened an ad hoc committee or experts who reviewed the correot Iiterature 00

lifting. recommend criteria for defining lifting capacity, and in 1991 developed a revised lifting equation. Subsequently, NIOSH developed the documentation for tbe equation and played a prominent role in recommending methods for interpret­ing tbe results of tbe equation. The 1991 equation reftects new findings and pro­vides metbods for evaluating asymmetrical lifting tasks, lifts of objects with less tban optimal hand--container couplings, and also provides guidelines for a larger range of work durations and lifting frequencies tban tbe 1981 equation. This paper provides tbe basis for selecting the three criteria (biomechanical, physiological, and psychophysical) tbat were used to define tbe 1991 equation, and describes the derivation of tbe individuai components (Putz-Anderson and Waters 1991). The paper also describes tbe Hfting index (LI), an index of relative physical stress, that can be used lo identify hazardous lifting tasks. Although the 1991 equation has not been fully validated, tbe recommended weight limits derived from the revised equation are consistent witb or lower than those generally reported in tbe literature. NIOSH believes that tbe revised 1991 lifting equation is more likely than tbe 1981 equation lo protect most workers.

1. Introduction The Nationallnstitute for Occupational Safety and Health (NIOSH) lirst developed an equation in 1981 to assist safety and health practitioners evaluate lifting demands in the sagittal pIane (NIOSH 1981). The lifting equation was widely used by occupational health practitioners because it provided an empirical method for computing a weight Iimit for manuallifting. This limit proved useful for identifying certain Iiftingjobs that posed a risk to the musculoskeletal system for developing lifting-related low back pain (Liles and Mahajan 1985). Because the 1981 equation could only be applied to a limited number of lifting tasks, namely sagittal lifting tasks, the 1981 equation was revised and expanded in 1991 lo apply to a larger percentage of lifting tasks.

Tbe 1991 lifting equation reflects new findings, provides methods for evaluating asymmetrical lifting tasks, objects with less than opti mal hand-container couplings. and offers new procedures for evaluating a larger range of work durations and lifting

0014-0139193 SIO.QO © 1993 Taylor & Francis Ltd.

750 T. R. Waters el aI.

frequeocies than the earlier equation. The objective of both equations is lo prevenl or reduce the occunence of lifting-related low back pain (LBP) among workers. An additionaI benefil of this equation is the potentiaI lo reduce other musculoskeletaI disorders or injuries associated with some lifting tasks such as shoulder or arm pain (Chaffin et al. 1976).

Three criteria (biomechanicaI. physiologicaI. and psychophysicaI) were used lo define the components of the originaI and revised lifting equation (Putz-Anderson and Waters 1991). The presenl docurneol describes the rationaIe for selecting these criteria and demonstrates how they were used lo determine the equation vaIues. The documenl aIso discusses the limitations of the lifting equation and the use of a lifting index for ideotifying hazardous jobs.

ne limitations of the lifting equation are a resull of the small number of scientific studies related lo some key hypotheses, the typicaI uncertainties witb tbe conclusions of mosl of the scientific studies, and the inabilily of correnl clinicaI metbods lo characterize accurately tbe specific pathoanatomic cause of most cases of work -related low back pain or other work-related musculoskeletaI disorders. In generaI, when faced with uncertainties in tbe data. tbe 1991 comminee chose tbe most conservative (Le., mosl protective) approach.

1.1. Occupational factors associated with LBP ManuaI handling and lifting are a major cause of work-relaled LBP and impairment. LBP aIso can occur by direct trauma, a single exertion ('overexertion '), or polentiaIly as the resull of multiple exertions ('repetitive trauma') (pope et al. 1991). SeveraI otber work-related faclors including pushing or pulling activities, extreme postures such as forward ftexion, and cyclic loading (whole body vibration) are aIso associaled witb developmenl of LBP and impairmenl.

Low back paio also is common in work environrnents where no lifting or manuai handling activities occur. such as work in a predominantly sitting posture (Lawrence 1955). In addition, evidence exists tbal work-relaled psychologicaI stress and lifeslyle factors aIso may increase the risk of LBP and tbe subsequenl risk of prolonged impairmenl or desirability (Bigos et al. 1986, Frymoyer et al. 1980). Moreover, tbe revised lifting equation accounts for only a limiled number of lifting-relaled task faclors (seven in aIl), and tberefore does noI include adjuslmenls for many of tbese otber importanl faclors. Furthermore, tbe lifting equalion applies only lo lifting tasks in which two hands are used lo move tbe load.

Although the lifetime prevaIence of LBP in tbe generaI population is as high as 70%, work-related LBP comprise only a subsel of aIl cases of LBP in the population (Frymoyer et al. 1983, NationaI Safely Couneil 1990). In generaI, the fraction of LBP which is work-relaled is difficull lo delermine in many work settings. Brown (1973) and Magora (1974) indicaled thal specific lifting or bending episodes were relaled lo only aboul one-third of tbe work-related cases of LBP. Thus, even tbe prevention of aIl LBP due IO lifting will noI prevenl aIl episodes of work -relaled pain, or prevenl tbe common non-work-relaled episodes of LBP.

1.2. Background The pasl 15 years of research on lifting-related LBP and manuaI lifting have produced three findings witb substantiaI scientific supporto (I) manuaI lifting poses a risk ofLBP

Discipline

Biomechanical Physiological Psychophysical

Note:

Revised NlOSH equation

Table l. Criteria used to develop tbe lifting equations.

Design criterioo

Maximum disc compression force Maximum energy expenditure Maximum acceptable weight

Cut-off value

3·4 kN (770Ibs) 2·2-4·7kcaVmin* Acceptable to 75% of female

workers and about 99% of male workers

751

:I: Since tbe energy expenditure limit for a specific task depends OD tbe vertical height of tbe lift and tbe duration of continuous lifting. task-specific criteria are presented in table 3.

lo many workers; (2) LBP is more likely lo occur when workers lift loads Ihal exceed their physical capacities; and (3) the physical capacities ofworters vary subslantially'

1.3. Development and history 01 the 1991 lifting equation The 1991 lifting equation is paltemed after the 1981 equation in ils developmenl, formaI, and interpretalion (N10SH 1981). Both versions are the producI of ad hoc NIOSH committees of experts who reviewed tbe curreot literature OD lifting, met. discussed the existing crileria for detining lifting capacily, and developed a lifting equation. When the 1991 equation was developed, however, NIOSH staff prepared the documentation for tbe lifting equation and played a prominent role in recommending methods for interpreting tbe results of tbe equatioo.2

Tbe 1991 conunittee's deliberations represented a unique compromise betweeo empirical findings and expert judgment, particularly when results were contradictory. iocoosistent. or simply limited. The maio product of tbe 1991 committee was tbe revised NIOSH lifting equation thal appears in Appendix A.

2. Basi. for selecting the criteria Both the 1981 and 1991 lifting equations are based on three crileria deri ved from the scientitic literalUre and the combined judgmenl of experts from the tields of bio­mechanics, psychophysics, and work physiology (table I). In generai, the crileria chosen by the NIOSH ad hoc committees (1981 and 1991) were used as a basis lo develop an equation for determining a recommeoded weight limit for a specific task. The recommended weighl limil for a task represents a load value that nearly ali healthy workers could perfonn over a substantial period of time (e.g., up lo 8 h) withoul an increased rist of developing lifting-relaled LBP.

Several criteria were used to develop tbe equatioo because each lifting task imposes differeot biomechanical and physiological requirements 00 tbe worker. As a result. tbe limiting factor or criteria in each lifting task may vary. Tbe biomechanical criterioo limits tbe effects of lumbosacral stress, which is most important io infrequent lifting tasks. Tbe physiologieal eriterioo limits tbe metabolie stress and fatigue associ-

I Physical capacities include static and dynarnic strength as well as various anatomical and physiological capacities such as flexibility, cardiovascular (aerobic) capacity. and tissue tolerance and recovery capacities. 2The ad hoc 1991 N10SH Lifting Conunittee members included M. M. Ayoub. Donald B. Chaffin. eolin G. Drury, Arun Garg. and Suzanne Rodgers. NIOSH representatives included Vero Putz-Anderson and Thomas R. Waters (see NTIS 1991).

752 T. R. Waters el al.

Table 2. individuaI criterioo and equation comparisons.

Estimated criterion-based weight loads (kg)

Lifting· examples Biomechanica1- Physiologica1b Psychophysicalc

Taskl 24 >24 14 Task2 >24 >24 13 Task3 20 7 8 Task4 24 6 12

Notes: • each of me fOUT tasks are described in the Append.ix. Part C; • based OD 350 kg disc compressioin force; bbased OD 3·1 kcaVrnin for Tasks 1.2. and 4. and 2·2kcallmin for Task 3;

1991 equation

RWL

IO I3 6 4

cbased OD maximum weight of lift acceptable to 75% of females; Tasks 1-3 are based OD

Soook and Ciriello (1991) and Task 4 is based 00 Ayoub et al. 1978.

ated witb repetitive lifting tasks.3 The psychophysicaI criterion limits the worldoad based on tbe workers' perception of tbeir lifting capability, a measure applicable lo nearly ali lifting tasks, excepl high-frequency lifting (above 6 Iifts per min).

Ideally, tbe criteria chosen IO establish tbe lifting equation should be based on a scientifically supported, quantitative relationship belween tbe criteria and tbe actual risk of lifting-relaled musculoskeletal injury or LBP. Since Ibis approach is noI currently feasible, tbe lifting criteria, for the mosl part, are based on secondary or surrogate measures of injury or LBP. For each of tbese secondary measures, there is a variable arnount of scientific or semi-quantitative evidence to indicate that the chosen lifting criteria can reliably predict the risk of Iifting-related LBP.

Because each criterion focuses OD different aspects of lifting stressors, recOID­

mended load weights that meet ODe criterion may Dot meet tbe otbers. For example. metabolic data suggest tbal it is more efficienl to Iift heavier weights less frequently tbat to lift lighter weights more frequenlly; however, biomechanical studies suggest tbat tbe load should be minimized by lifting Iighter weights more frequently to reduce muscle and vertebral stresses. Furthermore when lifting from tbe lloor, tesults from psychophysical studies suggesl tbat workers can typicaIly lift heavier loads tban tbose estimated from biomechanical or physiological studies. Hence. load recommendations for lifting often vary depending on which criteria are applied.

Because each criterion may provide a unique load limit for a specified lifting task. tbe 1991 comminee designed tbe lifting equation lo provide, in generaI, the most conservative load limit allowed by any individuai criterion.

An example of Ibis approach is provided in table 2. The detalIs of how tbe values were determined is provided in tbe Appendix , Part C. In table 2, estimated load Iimits are presented for four sample lifting tasks tbal are based solely on each criterion. The last colurno shows tbe 1991 equation values, which as Doted, are lower tban values based on tbe individuai criterion. As discussed in section 7. the lower recommended weight Iimit values are primarily attributed to tbe multiplicative nature oftbe equation.

Differences belween tbe physiologically-based weights and tbe recommended weight limil (RWL) values vary depending on how many factors are drawn into tbe

] The effects of Iocal muscle fatigue are discussed in section 4.

Revised NIOSH equation 753

equation (i.e., frequency. asymmetry. vertical faetar. etc., as required lo analyse tbe lifting task).

3. Biomechanical criterion Three issues underlie the 1991 committee's selection of the biomechanical criterioo for the NIOSH lifting equation: (1) the.choice of joint between the L5 and S I vertebra! segments (L5/S I) as the site of greatest lumbar stress during lifting; (2) the choice of compressive force as the criticai stress vector; and (3) the decision to select 3·4kN as tbc compressive force that defines aD increased risk of low-back injury.

3.1. Site oJ grealest lumbar stress during lifting An established biomechanical hypothesis is that the capacity for infrequent Iifts is a combined function of tbe individual's musc1e strength and tbe strength of various body structures, particularly the lumbar spine. Sludies have conlirmed thal lifting uoder certain conditions is limited more by the stresses OD tbe lumbar spine than by Iimitations of strength (Chaffin and Moulis 1969). Moreover, when manual lifting is modelled,large moments are crealed in the trunk area, especially when the load cannol be held c10se lo the body (Chaffin and Andersson 1984). Because the disc belween L5 and SI vertebrae has the potenlial lo incur the grealesl momenl in lifting and is also ODe of tbe most vulnerable tissues lo force-induced injuries, many investigators have 80ught lo obtain estimates of tbe biomechanical stresses for the L5/S 1 disc (Chaflin 1969, Tichauer 1971, Krusen et al. 1965, Garg et al. 1982, Anderson et al. 1985).

3.2. Compressive force as lhe criticai stress vector During lifting, three types of stress vectors are transmitted through tbe spinal muscu­loskeletal tissues lo tbe L5/S 1: compressive force, shear force, and torsional force. Tbe relative importance of each stress vector is nol wellunderstood. Disc compression is believed lO be largely responsible for vertebral end-plale fracture, disc hemialion, and resulting nerve rool irrilalion (Chaflin and Andersson 1984). Moreover, large compression forces al tbe L5/S l spinal disc can be produced by muscular exertion. especially during lifting (Chaflin and Andersson 1984). Herrin et al. (1986) concluded thal 'tbe biomechanical criterioo of maximal back compression appears lo be a good predictor not aoly of risk of low-hack incidents but of overexertion injuries in generai' . Because of tbe clinica) interest in disc diseases and their causes. numerous studies have been conducted lo assess tbe compressive strength of the tumbar vertebra! bodies and intervertebral discs. As a result of these and similar findings, and tbe accompanying uncertainty regarding tbe effects of shear and torsional stresses on lumbar tissue. disc compressive force was chosen by tbe 1991 committee as tbe criticai stress vector underlying tbe biomechanical criterion used to develop tbe lifting equation.

3.3. Determining the compressive force that dejines increased risk Because in vivo measures of compressive force are difficult, if not impossible. to undertake with current technology. tbe 1991 committee reviewed data from cross­sectional field studies that provided estimates of compressive forces generated by lifting tasks and subsequent injuries. Ultimately. prospective studies are needed to identify compressive force levels at tbe L5/S l joint tbat increase risk of low-back injury.

754 T. R. Waters el al.

3.3.1. Cadaver data: These data bave been used lo evaluate tbe strength of lumbar specimens lo witbstand applied compressive force. Witb data collected fO! 307lumbar segments from various studies, lager and Luttman (1989) determined tbe compressi ve strength of tbe lumbar segments and found a mean value of 4·4 kN witb a standard deviation of 1·88 kN. These results suggesl tbal if tbe data were normally distributed, approximately 30% of tbe lumbar segments had an ultimale compressive strength of less tban 3·4kN and 16% had an ultimate compressive strength of less tban 2·5kN (I standard deviation less tban tbe mean). Since tbe distribution pattern of data was noI provided, however, we cannol accùralely predicI tbe percentage of lumbar segments with maximum compressive sttength values less than 3·4kN.

Brinckmann et al. (1988) found maximum compressive strength values for vertebra! segments ranging from 2·1 lo 9·6 kN. The data indicale tbal fewer tban 21 % of tbe cadaver spinal segments fractured O! experienced end-plale failure al loads below 3·4kN, whereas only one segmenl failed al loads below 2·5kN.

Cadaver studies generally show large variability in tbe measured compressive strength of!be spine within and belween studies. This may be due lO declines in lumbar strengtb witb age, bone mineral contenI, and degenerative cbanges (Hansson and Roos 1981). Typically, tbe data showed tbal as tbe compressive force on tbe spine increased, tbere was an increase in tbe percentage of vertebra which were damaged. Por a small fraction of vertebra, damage occunred al compressive force levels as low as 2·5 kN. One of tbe limitations of tbe vertebra compressive strength data is uncertainty whelher compression injury 10 vertebra in cadaver studies is a reliable predictor of tbe risk of Iifting-relaled low back pain, impairmenl, or disabilily.

3.3.2. Biomechanical models: These models have been used lo estimate in vivo com­pressive forces on tbe LS/SI intervertebral joinl and disc. Chaflin (1969) developed one of tbe lirsl widely applied biomechanical models, based on a relinemenl of tbe Morris et al. (1961) static sagittal-plane (SSP) model. Chaflin's model incIuded only Iwo sourees of inlernal forces for resisting tbe extemalload momenl of lifting: (I) tbe action of tbe extensor ereclor spinae muscle; and (2) tbe stabilizing force provided by tbe pressure of tbe abdominal cavity. The model predicted compressive forces for tbe lumbosacral disc. These predicted forces were based on tbe weighl of tbe load and its distance from tbe base of tbe spine. More complex biomechanical models bave been developed, bUI each model requires specilic assumptions and simplilications (Gracovetsky and Farfan 1986, McGilI and Norman 1986, and Bean et al. 1988). In general~ each mode) provides somewhat different estimates of spina! compressive forces.

In tbe future. compressive forces may be predicted more accurately by biomechan­ica1 models that consider tbe dynamic components of lifting. possible antagonistic muscle forces, passive tissue loading, and tbe three dimensionalloading characteristics of tbe muscles. The dynamic componenl of lifting may be especially importanl for understanding tbe cause of back injury. Specilically, a number of investigalors have reported tbal Iifts witb ltigh acceleration components produce greater predicted com­pressive forces 00 tbe spine than lifts in which tbe acceleration is assumed to be zero. The estimaled compressive values for !be dynamic models ranged from 19% lo 200% greater tban tbe static model predictions (Garg et al. 1982, Leskinen et al. 1983, Preivalds 1984, McGilI and Norman 1985, Bush-loseph et al. 1988, Marras and Sommerich 1991a, 1991b). Because!be 1991 committee lacked data Iinking tbe pre-

Revised NIOSH equation 755

dicted dynamic compressive forees to the observed incidence of lifting-related LBP, the committee chose the simpler and older model to develop the foree criterion for disc compression.

Four studies have reported a direct relationship between lifting-related LBP and predicted static compressive foree on the L5/S I disc (Herrin et al. 1986, Bringham and Garg 1983, Anderson 1983, Chaffin and Park 1973). In a retrospective study, Herrin et al. (1986) eva1uated 55 industriaI jobs using a biomechanical mode!. Tbe study sample consisted of 2934 potentially stressful manual materials handling tasks. Tbe investigators traced tbe medicai reports of6912 incumbent workers employed in these jobs. For jobs with predicted compressive forces between 4·5 kN (1000lb) and 6·8kN (1500lb), the rate ofback problems was more than 1·5' times greater than that for jobs with compressive forees below 4·5 kN.

In another study, Bringham and Garg (1983) reported thatjobs in which workers experienced muscular strains had an average estimated compressive force of 5·34kN. Furthennore. jobs in which workers had disc injuries had an average estimated com­pressive foree of 7·97kN. In a similar study, Anderson (1983) reported that when males performed lifting jobs with a predicted compressive force exceeding 3·4kN, they had a 40% higher incidence rate of LBP than did males employed in jobs with predicted compressive forces below that leve\. Chaffin and Park conducted a similar study relating compressive force to injury incidence. as cited in tbe Wark Practices Guide for Manual Lifting (NIOSH 1981). A1though their study cannot be used to determine tbe difference in injury incidence rates for jobs with compressive forces above and below 3·4 kN, they suggested that (1) the LBP incidence for repetitive lifting tasks was less than 5% when tbe predicted compressive force OD tbe L5/S l joint was below 2·5 Kn, and (2) the incidence rate increased to more than 10% when the pre­dicted compressive foree exceeded 4·5 kN.

3.4. Biomechanical conclusions The 1991 committee recognized tbe limitations and uneertainties of biomechanical modelling of tbe lumbar spine. Even tbe most complex models only provide estimates of tbe relative magnitude of tbe compressive farce rather tban provide reliable esti­mates of absolute foree levels. In generaI. tbe committee based its finaI determination for the biomechanical criterion (i.e., 3·4 kN) on data from field studies in which some quantitative data were provided linking compressive force estimates witb tbe incidenee of low-back disorders. Given the limitations and variability of the data Iinking com­pressive foree and injury incidence, the 1991 NIOSH committee decided to maintain the 1981 biomechanical eriterion of 3·4 kN compressive force far its revision of tbe 1991 lifting equation.

3.5. N10SH perspective Tbe NIOSH perspective independent of the 1991 committee, is that a maximum compressive force of 3·4 kN on tbe L5/S 1 vertebrae may not proteet tbe entire work­foree for two principal reasons: (I) data from some of the workplace studies suggest tbat even in survivor workplaee populations, Jobs with compressive forees below

4 In tbe published artiele, tbe incidence rate of back problems for jobs witb maximum back compression between 4·5kN and 6·8kN was incorrectly reported as 1091200,OOOh or 18 times tbe rate far jobs with disc compression below 4·5kN. The aeroal rate was 91200,OOOh, or 1·5 times the rate for jobs witb maximum disc compression farce below 4·5 kN (based on personal correspondence with tbe NIOSH project director for this study).

756 T. R. Waters el al.

34 kN were associated witb an increase in tbe risk of back injuries; and (2) data from laboratory cadaver studies indicate that some members of tbe general population may suffer end-plate failure when performing Iifts tbal create compressive forces below 34kN.

4. Physiological criterion The 1991 commiltee selected tbe physiological criterion of energy expenditure lo Iimil loads for repetitive lifting. A main reason is that dynamic activities 5uch as walking. load carrying, and repealed load lifting use more musele groups tban infrequenl lifting tasks. Because tbe aerobic energy demands of dynamic lifting tasks require multiple musele groups lo move botb tbe load and tbe body, large energy expenditures are required lo supply tbe museles witb sufficienl oxygen for contraction. Witbouloxygen lo release adenosine triphosphale (A TP), prolonged dynamic activity cannol be sus­tained. Wben tbe metabolic demands of dynamic and sustained activily exceed tbe energy producing capacity of a worker. muscle contractioo is affected and whole body fatigue is usually experienced (Astrand and Rodahl 1986).

Since il is assumed that tbe lifts are made within a 3 s lime frame. local muscle fatigue should noI develop. Moreover, local muscle fatigue tbal could develop from high-frequency repelitive lifting or from heavy workloads is Iimited by tbe values in tbe frequency multiplier table tbal are provided witb tbe equation (Rodgers et al. 1991). Heavy workload is defined as museular exertion > 70% of maximum voluntary contractioo.

Altbough tbere is limiled empirical data demonstraling tbal whole body fatigue increases tbe risk of museuloskeletal injury, tbe 1991 comminee recognized tbal repetitive lifting task:s could easi1y exceed a worker's Dannai energy capacities. causing a premature decrease in strength and increasing tbe likelihood of injury (Lehmann 1958, Brown 1972, Garg and Saxena 1979). To control excessive fatigue, a baseline maximum aerobic capacity was established lo detennine maximum expen­diture for repetitive lifting tasks. A criteria desigued lo Iimil excessive whole body fatigue, however. does nol necessarily protect against tbe potentially hazardous cumulative effects of repetitive lifting.

Three important decisions underlie tbe 1991 committee's selection ofthe baseline maximum aerobic capacity and resllitant limits for task specific energy expenditures: (1) tbe choice of 9·5 kcallmin as tbe baseline measure of maximum aerobic lifting capacily used lo delermine tbe energy expendilure limits for repetitive lifting tasks; (2) tbe choice of tbe percentage (70%) of baseline maximum aerobic capacity used to establish an energy expenditure Iimil for lifts tbal predominantly require arm work (i.e., lifts above 75 cm or 30 inches); and (3) tbe choice of tbree percentages (50%, 40%, and 33%) of baseline maximum aerobic lifting capacily lo eslablish energy expenditure limits for lifting tasks lasling I h, I lo 2 h, and 2 lo 8 h, respectively.

4.1. Rationale for the baseline maximum aerobic capacity Aerobic capacity varies widely among workers according to age. sex, physical fitness. elc. (Astrand and RodahI1986). Average maximum aerobic capacilies, assessed using treadrnill procedures. have been reponed for 20-year-old condilioned male workers lo be as high as 20 kcaVmin and as"low as 7·3 kcaVrnin for 55-year-old female workers (Astrand and Rodahl 1986, Coleman and Burford 1971). In generai, older workers bove a lower capacily tban younger workers, and female workers have a lower capacity

Revised NIOSH equation 757

Table 3. Task-specific energy expenditure limits for frequent lifting (kcallmin).

Duration of lifting Lift location (V) cm (in.) <Ih 1-2h 2-8 h

V" 75 (30) 4·7 3·7 3·1 V> 75 (30) 3·3 2·7 2·2

than male workers. To a moderate extent, physical conditioning also rnay increase an individua!'s aerobic capacity to perfonn repetitive lifting (Astrand and Rodabl 1986).

In order to determine energy expenditure !imits for repetitive lifting as shown in table 3. the 1991 committee selected a baseline maximum aerobic capacity that could be adjusted to accommodate different lifting conditions. Most existing measures of maxirnum aerobic capacity were obtained from subjects using a treadmill test. According to Petrofsky and Lind (l978a, 1978b), however, the maximum aerobic capacity measures obtained using a treadrnill test overestimate the maximum aerobic capacity available for performing repetitive lifting tasks (Rodgers et al. 1991). As a result, the 1991 committee reduced the baseline aerobic capacity from the 1981 value of 10·5 kcaUmin to 9·5 kcaUmin to adjust for the difference between treadmill data and data collected from manual lifting studies. (A value of 9·5 kcaUmin is equivalent to a capacity of 4000 kcal per day for a 420min period of work.) The 1991 committee selected this value as the assumed mean aerobic lifting capacity of the average (50th percentile) 4O-year old female worker (Eastman Kodak 1986). This baseline aerobic capacity was subsequent1y adjusted for various lifting locations and durations of repetitive lifting (table 3 and Appendix B).

Although the 1991 committee chose a physiological criterion that represented the capacity of a 50th percentile female, rather than the capacity of the 75th percentile female. they were not necessarily endorsing a 50th percentile criterion. Tbe committee recognized that the multiplicative nature of the equation would provide a final weight limit that would be lower than a weight limit generated solely on the basis of the 50th percentile female physiological criterion. Their decision seerns to be appropriate considering tbe effects of the other factors in tbe equation. For example, tbe RWL values forthe repetitive tasks in !able 2 (Tasks 3 and 4) are lower than the weight limits derived solely from the physiological criterion ..

The committee's raionale for choosing tbe physiological criterion also was based on the belief that: (I) workers often can vary their lifting pace; and (2) vary their activities to reduce accumulated fatigue (Rodgers et al. 1991). Hence. in situations in which workers are unable to exercise some control over their rate of work, tbe recom­mended weight Iirnits for repetitive lifting jobs could be excessive for workers who are not well conditioned, leading to both Iaeal and systemic fatigue

Further research on paced lifting is needed to determine if tbe revised lifting equation is suitable for such conditions.

4.2. Rationale for task-specific energy expenditure limits 4.2.1. Adjustments for vertical lifting locations: Whole-body work is required when lifts are below waist level (i.e., when they involve the leg, low back, shoulder, and arm museles, such as when V < about 75 cm or 30 in), but lifts above waist level require primarily tbe shoulder and arm muscles. Since an ann tifi requires less muscular

758 T. R. Waters el al.

activity than a whole body lift, the maximum energy expenditure also is less for an arm tift. However, the maximum aerobic capacity for arm work is also lower (about 70%) than !haI attained for whole-body aerobic activity (Astrand and Rodahl 1986, Shatp et al. 1988). Hence, both work capacily and energy expenditure are reduced for arm tifts. As a resull, the 1991 committee recommended a 30% reduction in the energy expenditure Iimit of 9·5 kcaVmin for lifting acts involving primarily the upper body (Le. V>75cm or 30in).

4.2.2. Adjustments for durations of repetitive lifting: To avoid high levels of whole­body fatigue, the 1991 committee concluded that the energy expenditure for repetitive lifting must also be based on Iimits that apply lo the duration of the taSk. Most studies and reviews recommend work Iimits of approximately 33% of the maximum aerobic capacity for repetitive lifting taSks that are longer than two hours (Asfour et al. 1988, Karwowski and Yates 1986, Legg and Pateman 1984, MitaI 1984a, Williams et al. 1982).

To adjusl energy expenditure values for the aerobic demands posed by different durations of repetitive lifting taSks, the 1991 committee selected the following Iimits: (I) Repetitive lifting taSks /asting l h or less should not require workers to exceed 50% of the 9·5 kcaVmin baseline maximum aerobic capacity value; (2) repetitive lifting taSks /asting l IO 2 h should noi require workers lo exceed 40% of the 9·5 k1caVmin baseline; and (3) repetitive lifting taSks /asting 2 to 8 h should noi require workers lo

exceed 33% ofthe 9·5kcaVmin baseline. The 1991 committee did noi provide energy expenditure Iimits for taSks lasting more than 8 h.

4.3. Physi%gical conclusions The goal of the 1991 committee was to prevenl systemic or aerobic fatigue and possibly local muscle fatigue thal might increase the risk of Iifting-related low back pain for a majority of physical/y fit workers engaged in repetitive manuaI lifting. As a resull, the 1991 committee computed the energy expenditure Iimits displayed in table 3, based on a maximum aerobic lifting capacity of 9·5 kcaVmin. Further research is needed lo validate the energy expenditure Iimits for the lifting conditions in table 3.

4.4. N10SH perspective The NIOSH perspective, independent ofthe 1991 committee, is!hal a baseline aerobic lifting capacity of 9·5 kcaVmin Iimil may be 100 high, particularly for older workers, since il could fail to prevent fatigue even in some healthy workers. Some studies indicate that both younger and older workers may bave maximum aerobic capacities below 9·5 kcaVmin. In generaI, the relationship between fatigue and risk ofback injury is noi sufficiently established lo determine precisely the level of excess risk for jobs thal exceed the energy expenditure limits in table 3. Additionally, the physiological criteria may not prevenl dysfunction or damage lo the tissues of the low back from the repetitive nature of lifting even if whole body fatigue is successfully prevented.

S. PsychnphysicaI "riterinn The psychophysicaI criterion is based on data defining workers' strength and capacity lo perform manual lifting al different frequencies for different durations. The psycho­physicaI criterion is defined directly by measures of maximum-acceptable-weight-of­Iift and indirectly from studies measuring isnmetric strength. Although strength is an importanl determinanl of the capability of an individuai lo perform an infrequenl or

Revised NIOSH equation 759

occasionaI lift, 'capability (maximum-acceptable-weight-of-lift) appears to be sub­stantially lower than isometric or isotonic strength maxima' (Ayoub and Milal 1989).

Tbe criticai issues for tbe psychophysical criterioo are as follows: (1) tbe rationale of the 1991 committee for choosing a criterion acceptable to 75% of female workers; and (2) the rationale for using maximum-acceptable-weight-of-lift and strength to detennine recommended weight limits.

5.1. Rationale for choosing the acceptability criterion Tbe maximum-acceptable-weight-of-lift is tbe amollnt of weight a persoo chooses to lift under given conditions for a defined periodo In measurements of maximum-accept­able-weight-of-lift. workers typically are asked to 'wOTk as hard as you can without straining yourself. or without becoming unusually tired. weakened. overheated. or out of breath' (Snook and Ciriello 1991). The maximum-acceptable-weight-of-lift provides an empirical measure that appears to integrate both biomechanical and physiological sources of stress for ali but certain high-frequency lifting tasks (Karwowski and Ayoub 1984). Unlike maximum strength measures, which define what a persoo can do 00 a single attempt. tbe maximum acceptable measure defines what a person can do repeatedly for an extended period without excessive fatigue. which may lead to lifting-related low back pain.

5.2. Re/ating maximum-acceptable-weight-of·lift to low back pain The 1991 committee selected the psychophysical criterion based on several studies that relate the incidence and severity of lifting-related low back pain to the extent to which lifting demands are judged acceptable to experienced workers. Specifically, injuries increased for lifting tasks rated acceptable by less than 75% to 90% of the workers (Snook 1978, Herrin et al. 1986). Snook (1978) summarized bis findings as follows:

The results revealed that approximately one-quarter of policyholder jobs involve manual handling tasks that are acceptable to less than 75% of the workers; however, one-halfofthe low back injuries were associated with thesejobs. This indicates that a worker is three times more susceptible to low back injury if performing a manual handling task that is acceptable to less than 75% of the working population. This also indicates that, at best, two out of every three Iow back injuries associated with heavy manual handIing tasks can be prevented if the tasks are designed to fit at least 75% of the population. The third injury will occur anyway, regardless of the job.

Several investigators reported that workers who have experienced back injury typically rate the physical effOrl in their jobs as greater than workers on similar jobs who have not had back injury (Magora 1970, Dehlin et al. 1976). Herrin et al. (1986) also reported !hat the rate of medical back incidents (Le., sprains, strains, degenerative disc disease, and other ill-defined pain) increased significant1y for jobs with strength demands that exceeded the lifting capability (i.e. the maximum acceptable weight) of 90% of the exposed workers.

The 1991 committee selected the psychophysical criterion to ensure that the job demands posed by manual lifting would not exceed the acceptable lifting capacity of about 99% of male workers and 75% of female workers--<>r 90% of the working population (if one assumes a working population that is 50% male and female).

760 T. R. Waters et al.

Table 4. Psychophysical and equation-based weight loads (kg).

Female per cent acceptability

Lifting tasks* 75% 90%

Small H. small V 18 16 H=37cm V= 78·5 cm Small H, large V 16 14 H=37cm v= 154cm Large H, .mall V 17 14 H=58cm V=78·5cm Large H, .malI V 12 \O H=58cm v= 154cm

Note: • Assuming FM, DM, AM, and CM are idealized (i.e., = I); Snook and Ciriello, 1991.

5.3. Psychophysical conclusions

1991 equation

RWL

15

12

\O

8

The p.ychophysical approach provides a method to estimate the combined effects of biomechanical and physiological stressors of manual lifting. Because it relies on self-reporting from subjects, the perceived 'acceptable' lintit may differ from the actual 'safe' lintit. Even though there is a relationship between the 'acceptable' and the 'safe' lintit, the psychophysical approach may nOI be equally valid for ali combinations of task variables. For example, most data indicate that the psychophysical approach overestimates workers' capacity for high-frequency lifting (> 6 liftslmin) (Ciriello and Snook 1983, Asfour et al. 1985, Karwowski and Yates 1986). TIte psychophysical approach also may overestimate capacity for lifting lasting more than about l h (Mital 1983). Fernandez and Ayoub (1987) and Ciriello et al. (1990), however, have recently refuted this concept. Fernandez and Ayoub found !hat the MA WL did noI decrease significantly over time. Ciriello et al. (1990) also found that psychophysical methods, when properly adntinistered, do not overestimate lifting capacity in tasks lasting up to four hours.

5.4. NIOSH perspective TIte NIOSH perspective, independent of the 1991 comntittee, is that the psycho­physical criterion of 'acceptability to 75% of female workers' does not!reat men and women equally. Nevertheless as shown in tables 4 and 5, the 1991 equation yields recommended weight lintits (RWLs) that are lower!han weights acceptable IO at least 90% of females. Hence, the 1991 equation provides a more equitable assessment of polentially hazardous lifting tasks for women than would be apparent from the psycho­physical criterion alone (i.e., acceptable IO 75% of females). For example, table 4 displays load weights (kg) from Snook and Ciriello (1991) for a series of typicallifting tasks involving variations in the borizontal (H) and vertical (V) factors. Also supplied are the corresponding RWLs computed from the 1991 equation. Ali four of the examples produced RWLs !hat were 10wer in weigbt than comparable psycbophysical values acceptable IO 90% of the females. In generai, the values provided by the 1991

Revised N10SH equation 761

Tabl.5. Comparisoo of recommended weight limits with Snook and Ciriello's maximum acceptable weight limit for 90% of femate workers. *

Snook and Ciriello's 1991

maximum Vertical Horizontal Vertical Recommended acceptable

dispiacernent distance of starting height weight limit weight Iimit for of tifi load from body of tifi (kg) 90% of femate (cm) (cm) (cm) RWL workers (kg)

Floor-knuckle 25 37 26 10·0 Il

45 26 8·2 9 58 26 6·3 9

51 37 12·5 8·7 Il 45 12·5 7·1 9 58 12·5 5·5 8

76 42 O 7·1 9 50 O 5·9 8 63 O 4·7 7

~uckJe-shoulder

25 37 92 Il-l 12 45 92 9·2 IO 58 92 7·1 IO

51 37 78·5 10·6 IO 45 78·5 8·7 9 58 78·5 6·7 9

76 37 66 10·0 9 45 66 8·3 9 58 66 6·3 9

Shoulder-reach 25 37 154 8·9 IO

45 154 7·3 8 58 154 5·6 8

51 37 141 8·5 9 45 141 7·0 7 58 141 5·4 7

76 37 128 8·7 8 45 128 7·1 7 58 128 5·5 6

Note: * Evaluated al a task frequency (F) of 1 liftlmin.

equation are consistent with or lower than tbe average lifting weights for task condi­tions reported by Snook and Ciriello. Tbose weight limits were acceptable lo 90% of the femaIes (table 5).

6. Derivation or the equation components Following the selection ofthe individuaI criterion, the 1991 committee developed the revised lifting equation (Appendix A). Tbis section presenls the derivation of tbe revised lifting equation and explains how tbe criteria were used to develop tbe individuaI componenlS. Tbe discussion addresses the standard lifting location, the load coostanl, and the derivation of the mathematicaI expressions (multipliers). Each

762 T. R. Waters el al.

componenl of tbe revised lifting equation (Appendix A) was designed lo satisfy tbe lifting criteria and was based, lo tbe extenl possible, on tbe results of quantitative research studies. Where tbe data were conflicting, however, decisions affecting tbe multipliers were based OD a consensus of tbe 1991 committee. In most cases. tbe final decisions represented tbe most conservative (i.e. the most protective) estimates of lifting capacity.

Tbe developmenl oftbe lifting equation required tbal: (I) a standard lifting location be defined; (2) a load constanl for tbe equation be established; and (3) tbe matbemalical expressions for each factor be derived.

6.1. Defining the standard lifting location The standard lifting location serves as tbe three-dimensional reference poiot for eval­uating tbe worker's lifting posture. Tbe standard lifting location for tbe 1981 equation was defined as a vertical heighl of 75 cm from tbe floor and a horizontal distance of 15cm from tbe mid-point between tbe anldes. Tbe 1991 equation continues to use a vertical heighl of75 cm for tbe standard reference location, as supported by recenl data (Ruhmann and Schntidtke 1989). However, !be horizontal displacemenl factor was increased from 15 lo 25 cm for tbe 1991 equalion. This increase reflects recenl findings tbal showed 25 cm as tbe ntinimum horizontal distance mosl often used by workers lifting loads tbal did noi interfere witb fronl of tbe body (Garg and Badger 1986, Garg 1986).

6.2. Establishing the load constant Tbe load constanl (23 kg or 51 Ibs) refers lo tbe maximum recommended weighl for lifting al tbe standard lifting location under optimal conditions (Le. sagitta1 position, occasional lifting, good couplings, ~ 25 cm vertical displacement, elc.). Selection of tbe load constanl is based on tbe psychophysical and biomechanical criteria. Tbe 1991 comntittee estimaled tbal lifting a load equivalenl lo tbe load constanl under ideai conditions (i.e., where ali of tbe factors are equal lo I·Q) would be acceptable IO 75% of female workers and aboul 90% of male workers and tbal !be disc compression force resulting from such a lifl would be less tban 3·4 kN.

For tbe revised equation, tbe load constanl was reduced from 40 lo 23 kg. This reduction was partIy driven by tbe need lo increase tbe 1981 ntinimum horizontal displacemenl from 15 to 25 cm for tbe 1991 equation, as noted above. Tbe revised load constanl is 17 kg less tban tbal for 1981; bui al tbe revised ntinimum horizontal displacemenl of 25 cm, tbe 23 kg load constanl represents only a I kg reduction from tbe 1981 equation when adjusled for revised horizontal distance. Tbis I kg reduction reflects recenl data reported by Snook and Ciriello (1991) indicaling tbal tbe maximum acceptable weighl lintil for female workers is lower tban tbe capacity tbal was reported in 1978 (Snook 1978).

Altbough tbe 23 kg load constanl was based on tbe maximum acceptable weighl lintil for 75% of female workers, tbe recommended weighl lintits are likely IO be acceptable IO al leasl 90% of female workers when tbe revised load constanl is applied in !be lifting equation. This conclusion is based on a comparison witb tbe Snook and Ciriello (1991) sludy (table 5).

6.3. Deriving mathematical expressions Tbe multipliers for tbe revised lifting equation refer IO tbe six coefficients (matb­ematical expressions) used IO reduce tbe load constanl IO compensate for character­istics of tbe lifting task which are differenl from tbe standard or optimal conditions

Revised NIOSH equation 763

(i.e., sagittal position, occasionai lifting, good couplings, ~ 25 cm vertical displace­ment, etc.). These conditions or factors were identified in one or more epidemiologie studies of manual lifting (Chaffin and Patk 1973, Snook 1978, Frymoyer et al. 1983, Bigos et al. 1986). Each of the six multipliers should satisfy ali three of the lifting criteria presented in table l. In most cases, the multipliers represent the most conser­vative estimate of lifting capacity for each individuai lifting factor.

The six multipliers (coefficients) were derived from a series of adjustments (itera­tions) in which the revised coefficients were used to generate predicted loads. These loads were then compared with empirically derived lifting values from the previously cited psychophysical lifting studies. The rationale for each of the six multipliers is briefty reviewed in the following subsections.

6.3.1. Horizontal multiplier: Biomechanical and psychophysical studies indicate that with increasing horizontal distance of the load from the spine, the predicted disc compression foree increases and tbe maximum acceptable weight limit decreases (Snook 1978, Chaffin and Andersson 1984, Garg 1986). The axial compression stress applied to the spine during lifting is generally proportional to the horizontal distance of the load from the spine. For example, both the load and the ftexion moment (the product of the load and the horizontal distance from the spinal axis) are important in deterrnining the axial compression stresses on the lumbar spine (Schultz et al. 1982, Chaffin and Andersson 1984). Furthermore, psychophysical data consistently indicate that as the load is moved horizontally from the spine, the amount of weight a person is willing to lift decreases proportionately (Snook 1978, Ayoub et al. 1978, Garg and Badger 1986, Snook and Ciriello 1991).

To satisfy the lifting criteria, the horizontal multiplier (HM) was deterrnined as follows:

HM=(25/H) (I)

where H = tbe horizontal distance in centimetres

HM=(I0/H) (2)

where H = the borizontal distance in inches

6.3.2. Vertical multiplier. Biomechanical studies suggest an increased lumbar stress for lifting loads near the ftoor (Chaffin 1969, Bean et ai. 1988). Epidentiologic studies indicate that lifting from near the ftoor is associated with a large percentage of low­back injuries attributable to lifting (Snook 1978, Punnel! et al. 1991). Physiological studies indicate that lifting from near the ftoor requires a significantly greater energy expenditure than lifting from greater heights (Fredrick 1959, Garg et al. 1978). Although no direct empirical data exist to provide a specific adjustment value for lifting near the ftoor, the 1991 comntittee recommended that the vertical factor provide at least a 22·5% decrease in the a1lowable weight for lifts originating near the ftoor. The rationale for reduction of loads lo be Iifted above 75 cm from the ftoor is based on empirical data from psychophysical studies indicating that a worker's maximum­acceptable-weight-<lf-Iift decreases as the vertical height of Iift (V) increases above 75 cm (Snook 1978, Ayoubetal. 1978, SnookandCiriello 1991). The 1991 comntittee chose a discount value of 22·5% lO decrease the a1lowable weighl for Iifts at shoulder level (l50cm, or 6Oin) and for Iifts at ftoor level, resulting in the following vertical multiplier.

764 T. R. Waters'et al.

VM = (1- 0·()()31 V -751) (3)

where V = vertical height in centimetres

VM = (1- 0·()()75 1 V - 301) (4)

where V = vertical height in inches

6.3.3. Distance multiplier. Tbe results of psychophysical studies suggest an approxi­mate 15% decrease in maximum-acceptable-weighl-of-Iift when !he lota! distance moved is near tbe maximum (e.g., lifts originating near tbe ftoor and ending above tbe shoulder (Garg et al. 1978, Snook 1978, Snook and Ciriello 1991). AIso, results of physiological studies indicate a significant increase in physiologica1 demand as tbe vertical distance of !he lift increases (Aquilano 1968, Khalil et al. 1985). Finally, for lifts in wbich !he lota! distance moved is < 25 cm ( < IO in), !he physiological demand is noI significantly increased, and !herefore !he multiplier should be held constanl. As a resull, !he distance multiplier (DM) was established by !he 1991 committee as follows:

DM = (0·82 + (4·5/D» (5)

where D = the totaI distance moved in centimetres

DM = (0·82 + (H/D» (6)

where D = the total distance moved in inches

6.3.4. Asymmetric multiplier: To dale, only a few studies provide data on !he relation­sbip between asymmetric lifting (Le., lifting loads away from !he sagittal piane) lo maximum acceptable lifting capacities. or !he limited number of psychophysical studies available, ali bave reported a decrease in maximum acceptable weighl (8% lo 22%) and a decrease in isometric lifting strength (39%) for asymmetric lifting tasks of 90 degrees compared wi!h symmetric lifting tasks (Garg and Badger 1986, Mital and Fard 1986, Garg and Banaag 1988). The results from biomechanical studies a1so suppor! a significanl decrease in !he allowable weighl for asymmetric lifting jobs (Bean et al. 1988).

Therefore, !he 1991 commiltee recommended !haI !he asymmetric multiplier be established so !haI !he a1lowable weighl of lift be reduced by aboUI 30% for Iifts involving asymmetric twists of 90 degrees. The asymmetric multiplier (AM) was established by!he 1991 committee as follows:

AM = (I - (0·()()32A» (7)

where A =!he angle belween !he sagittal piane and !he piane of asymmetry. (The asymmetry piane is defined as !he vertical piane !haI intersects !he midpoinl belween !he anldes and !he midpoinl between !he knuckles al !he asymmetric location.)

6.3.5. Coupling multiplier: Loads equipped wi!h appropriate couplings or handles facilitale lifting and reduce !he possibility of dropping !he load. Psychophysical studies !haI investigaled !he effects of bandles on maximum-acceptable-weighl-of-Iift sug­gested !haI lifting capacity was décreased in lifting tasks involving containe", wi!houl good bandles (Garg and Saxena 1980, Smi!h and Jiang 1984, Drury et al. 1989). AI!hough !bese studies did noI agree precisely on !he degree of reduction in lifting

Couplings

Good Fair Poor

Revised NIOSH equation

Table 6. Coupling multiplier.

V<75cm (30in) V;" 75 cm (30in)

Coupling multipliers

HJO 0·95 0·90

1·00 1·00 0·90

765

capacity, most concluded !hat!he reduction should be in!he range of about 7% to Il % for containers wi!hout handles. The coupling multipliers are displayed in table 6.

Considering !he quality of !he data and !he difficulty in judging !he quality of!he coupling, !he consensus of!he 1991 committee was !hat!he penalty for a poor coupling should not exceed 10%. Hence, !he container coupling multiplier (CM) was defined as follows:

CM = 1·0, 0·95, or 0·90 (8)

depending on !he vertical height of!he lift and !he quality of!he couplings. Coupling quality was categorized as good, fair, or poor. Height was categorized as ,; 75 cm (30 in) or > 75 cm.

6.3.6. Frequency multiplier. For!he 1991 lifting equation, the appropriate frequency multiplier is obtained from a table (table 7) ra!her !han from a mathematical

Table 7. Frequency multiplier (FM).

Work duration

,;lh ,;2h ,;8h Frequency liftslmin V<75 V;" 75 V<75 V;" 75 V<75 V;" 75

0·2 1·00 1·00 0·95 0·95 0·85 0·85 0·5 0·97 0·97 0·92 0·92 0·81 0·81 I 0·94 0·94 0·88 0·88 0·75 0·75 2 0·91 0·91 0·84 0·84 0·65 0·65 3 0·88 0·88 0·79 0·79 0·55 0·55 4 0·84 0·84 0·72 0·72 0·45 0·45 5 0·80 0·80 0·60 0·60 0·35 0·35 6 0·75 0·75 0·50 0·50 0·27 0·27 7 0·70 0·70 0·42 0·42 0·22 0·22 8 0·60 0·60 0·35 0·35 0·18 0·18 9 0·52 0·52 0·30 0·30 0·00 0·15

lO 0·45 0·45 0·26 0·26 0·00 0·13 Il 0·41 0-41 0·00 0·23 0·00 0·00 12 0·37 0·37 0·00 0·21 0·00 0·00 13 0·00 0·34 0·00 0·00 0·00 0·00 14 0·00 0·31 O.()() 0·00 0·00 0·00 15 ().OO 0·28 0·00 0·00 0·00 0·00

>15 0·00 0·00 0·00 0·00 0·00 0·00

Note: t values of V are in cm; 75 cm = 30 in.

766 T. R. Waters et al.

expression and table. as was !be case in tbe 1981 lifting equation (i.e .• in 1981. tbe FM = I - [FIF_l. where FM = !be frequency multiplier. F = task frequency rate. and F _ = maximum frequency as obtained from a table).

11te frequency multipliers in table 7 are based on two sets oC data. For lifting frequencies up to 4 lifts/min. psychophysical data from Snook and Ciriello (1991) were used to develop !be frequency multiplier (FM) values.' 11tese FM values are shown in !be upper portion oC table 7 (alI cells in tbe first six rows).

For lifting frequencies above 4 lifts/min. tbe frequency multipliers values. which are displayed in table 7. row 5 and below. were determined from a three-step process using !be energy expenditure prediction equations developed by Garg (1976) (Garg et al. 1978) (see Appendix. Part D).

The first step used Garg' s empirical1y-derived linear regression equations to predict tbe energy demands oC lifting tasks for frequencies above 4 lifts/min. 11te equations include terms for gender. weight of loa(\, frequency of lifts. and tbe worker' s body weight. Two equations were used, one for lifts below tbe waist and one for lifts above tbe waist, namely: a stoop-lift equation and an arm-lift equation (Rodgers et al. 1991: 34-35). Assuming a body weight of l30\bs for a woman. Garg in an iterative approach determined tbe combinations of frequencies of lifts and weights of loads tbat would yield energy expenditure values equivalent to tbose in table 3. For alI calcula­tions? (be most energy efficient lifting posture was assumed since workers tend to use tbe most efficient metbod.

In tbe second step. frequency multipliers were !ben generated from tbese inter­mediate load weights tbat would provide Recommended Weight Umits equivalent to !be load weights determined from !be first step.

For !be third step. tbe committee reviewed and adjusted tbe frequency multipliers in table 7 to ensure that: (1) !be frequency multipliers for Iifts below 30 incbes would not exceed tbose for lifts of 30 incbes or above; and (2) tbat tbe transition zone between tbe psychophysical- and physiological-derived frequency multipliers (i.e .• 41ifis/min) provided continuous values. In generai. the frequency multiplier values in table 7 meet !be energy criteria provided in table 3 witb a few exceptions. The results oC tbe analysis are provided in greater detail in Rodgers (1991: 35-37).

11te committee did note in tbeir analysis. however. tbat tbe energy expenditure for repetitive squat Iifts may exceed tbe energy expenditure limits listed in table 3. row l. Tbis finding is also consistent witb different studies showing tbat tbe energy demands for squat postures are greater tban for stoop postures (Frederik 1959. Garg and Herrin 1979. Kumar 1984).

11te committee concluded tbat tbe frequency multipliers provide a c10se approxi­mation of observed and predicted effects of lifting frequency on acceptable workloads for lifting (Rodgers et al. 1991: 37).

From !be NIOSH perspective. it is possible tbat obese workers may exceed tbe energy expenditure criteria for lifts from below tbe waisL In addition. tbere are some circurnstances in whicb Ioca\ muscle fatigue may occur even tbough whole body fatigue has noi occurred. Tbis is most Iikely in situations involving lifting al high rates for longer !han 15 min. or prolonged use of awkward postures. sucb as constant bending.

!I Snook md Ciriello's (1991) data provide n:conunended wcight limits for repetitive manuallifting ~ks perfonned under a wide variety of conditions (diffcrent beight:s. Iocations, &Dd hquencies).

Revised NIOSH equation 767

7. Identlfylng hazardous lifting jobs with the lifting index A key eoncepl of \he 1981 lifting equalion is \haI \he risk of lifting-relaled low baek pain increases as the demands of \he lifling task inerease (Chaffin and Park 1973, Snook 1978. Hemn et al. 1986). Based on this eoneepl, \he 1981 lifting equalion was used lo define Iwo points: \he aclion limil and \he maximum pennissible limil (whieh is three limes \he aelion limit)o For job assessmenl putposes. lifting jobs \haI required workers lo lift loads below \he aclion limil were eonsidered lo pose little risk of lifting-relaled low baek pain for mosl workers. Liftingjobs \haI required workers lo lift loads between \he aelion limil and \he maximum pennissible limil likely pose in­ereased risk for some workers buI noI for o\hers. And lifting jobs \haI required workers to tifi loads above tbe maximum pennissible limit were considered lo pose a significant risk of lifting-relaled low back pain for many workers.

The 1991 equalion is also based on \he eoneepl \haI \he risk of lifting-related low baek pain inereases as \he demands of \he lifting task increase. Ra\her \han using a three-stage decision matrix, however, as was used wi\h \he 198 I equalion, a single lifting index (LI) was proposed for \he 1991 equalion. Specifieally, \he LI is \he ralio of \he load lifted lo \he reeommended weighl limil. The lifting index (LI) is similar in eoneepl lo Ayoub'sjob severity index (JSI) and Chaffin's lifting streng\h raling (LSR) (Ayoub et al. 1978 and Chaffin 1974). Eaeh of \hese indiees eneompass \he nolion \haI tbe risk ofinjury increases as tbe ]oad or job demands exceeds some baseline capacity of tbe worker. This capacity may be estimated from a lifting equation. or from esti­males ofworker's streng\h. as assessed by various psyehophysical tests and regression models.

The lifting index (LI) provides a simple melhod for eomparing \he lifting demands associaled wi\h differenl lifling tasks in whieh \he load weights vary and \he recom­mended weighl limits (RWL) vary. In \heory, \he magnitude of\he LI may be used as a gauge lo eslimale \he percentage of Ihe workforce \haI is likely lo be al risk for developing lifting-relaled low baek pain. The shape of Ihe risk funelion, however, is noI known. Thus il is noI possible lo quanlify \he precise degree of risk associated wi\h increments in the lifting index. In a similar manner, there is uncertainty about whether a lifting index of ODe is a reliable boundary for differentiating between an increase in risk and no increase in risk for some &actioo of tbe working population. The previous discussion of tbe criteria underlying tbe lifting equation and of the equation multipliers highlight the assumptions and uncertainties in the scientific studies and the theoretical models which have related lifting to low back injuries. However, these uncertainties do noI ali poinl in \he same direclion. Some support \he belief \haI a lifting index of one will pIace a substantial fiaction of the work force at an increased risk of low back pain. Others support \he belief \haI mosl of \he work force ean work safely above a lifting index of one.

Three of the most important limitations of the equation are the following:

(I) A signifieanl part of \he equalion is based on psyehophysieal laboralory studies. Since these data are obtained from workers' judgment of perceived lifting stress, psychophysical data may reveal more about a worker' s tolerance IO stress \han of impending low baek pain.

(2) The physiological criterion is based on restricting energy expenditures to avoid whole body faligue. The eriterion, however, does noI address \he polenlial risk associated with the cumulative effects of repetitive lifting, which may be independenl of \he level of whole body faligue.

768 T. R. Waters et al.

(3) If tbe three criteria for tbe equation were considered individually, tbey would probably not be protective of alI workers.

A main tenet of our approach, however, is tbat tbe multiplicative nature of tbe equation has provided a final equation tbat is more likely to protect bealtby workers tban each individuai criterion. Specifically, when several factors deviate from tbe ideai (i.e., standard lift location), tbe decline in tbe predicted value obtained from a multi­plicative model for most lifts depends on tbe product of several factors; this substan­tially reduces tbe RWL. Based on individuai parameters, tbe multiplicative model defines discrete regions where no lifting is allowed no matter how ideai tbe otber parameters are. For example, if tbe horizontal factor exceeds 25 inches, tbe multiplier is zero, resulting in a computed RWL value of zero. This means tbat no weight should be lifted for this task condition.

Despite tbe limitations of tbe research studies and inherent uncertainties in relying on exper! judgment, it is likely tbat lifting tasks witb a lifting index > I pose an increased risk for lifting-related low back pain for some fraction of tbe workforce. Therefore, tbe lifting index may be used to identify potentially hazardous lifting jobs or to compare tbe relative severity of two jobs for tbe purpose of evaluating and redesigning tbem.

Some members of tbe 1991 committee believe tbat worker selection criteria based OD research studies~ empiricaI observations. or theoretica1 considerations such as job­related strength testing or aerobic capacity testing can accurately identify workers who can perform lifting tasks witb a lifting index > I witbout an increased risk of a work-related injury (Chaffin and Andersson 1984, Ayoub and Mital 1989). These members agree, however, tbat many workers will be at elevated risk iftbe I;fting index exceeds 3·0. Additionally, some members of tbe 1991 committee believe tbat tbe 6informal' selection of workers which occurs in many jobs that require repetitive lifting tasks lead to a workforce tbat can work above a lifting index of 1·0 witbout substantial risk of low back injuries above tbe baseline rate of injury.

8. Llmltatlons or tbe 1991 lifting equation 8.1. Generai limilations The lifting equation is a specialized risk assessment tool. As witb any specialized tool, its application is Iimited to tbose conditions for which it was designed. Specifically, tbe lifting equation was designed to meet select lifting-related criteria tbat encompasses biomechanical, work physiology, and psychophysical assumptions and data. identified above. To tbe extent tbat a given lifting task accurately reftects tbese underlying conditions and criteria. tbis lifting equation may be appropriately applied. The follow­ing Iist identifies a set of work conditions in which tbe appliçation of !be lifting equation would either under-or-over estimate tbe risk of low back paio or injury. Each of tbe following task limitations also highlight research topics in need of further research to extend tbe application of tbe lifting equation to a greater range of real world lifting tasks.

I. Tbc 1991 lifting equation assumes tbat manual handling activities otber tban lifting are minimal and do not require significant energy expenditure, especially wben repetitive lifting tasks are performed. Examples of non-lifting tasks include holding, pushing, pulling, carrying, walking, and c1imbing. If such non-lifting activities are common, measures of workers' energy expenditures and bear! rate may be required to assess tbe metabolic demands of tbe different tasks.

Revised NIOSH equation 769

2. The 1991 lifting equation does not include task factors to account for unpredicted conditions, such as unexpectedly heavy loads, slips, or falls. Additional biomechanical analyses may be required to assess the physical stress on joints that occur from traumatic incidents. Moreover, if tbe environment is unfavourable (e.g., temperature or humidity significantIy outside the range of 19° to 26°C [66° to 79°F] or 35% lo 50%, respectively) independent metabolic assessments would be needed to gauge tbe effects of tbese variables on heart rate and energy consumption.

3. The 1991 lifting equation was not designed to assess tasks involving one­banded lifting, lifting while seated or kneeling, lifting in a constrained work space, lifting people, lifting of extremely bol, cold, or contaminated objects, lifting of wheel barrels, sboveling, or high-speed lifting (i.e., lifting that is not performed within a 2-4 s time frame). For such task conditions, independent and task specific biomechanical, metabolic, and psychophysical assessments are needed.

4. The 1991 lifting equation assumes that the worker/floor surface coupling provides at least a 0·4 (preferably 0·5) coefficient of static friction between the shoe sole and the working surface. An adequate worker/floor surface coupling is necessary when lifting to provide a firm footing and to control accidents and injuries resulting from foot slippage. A 04 to 0·5 coefficient of static friction is comparable to the friction found between a smooth, dry floor and the sole of a clean, dry leather work shoe (nonslip type). lndependent biomechanical modelling may be used to account for variations in tbe coefficient of frietion.

5. The 1991 lifting equation assumes that lifting and lowering tasks have the same level of risk for low back injuries (i.e., that lifting a box from the floor to a table is equally as hazardous as lowering the sarne box from a table to the floor). This assump­tion may not be true if tbe worker actually drops or guides tbe box to tbe ftoor ratber than lowers alI tbe way lo the ftoor. Independent psychophysical assessments need to be undertaken to assess worker capacity for various lowering conditions.

In conclusions, the lifting equation is only one tool in a comprehensive effort to prevent work-related low back pain and disability. Lifting is only one ofthe causes of work-related low back pain and disability. There are many other causes which have been hypothesized or established as factors including whole body vibration, static postures, prolonged sitting, and direct trauma to the back. Psychosocial factors, appro­priate medicai treatment, and job demands also may be particularly important in inftuencing the transition of acute low back paio to chronic disabling paio.

8.2. The need for validation AlI methods need validation. For the 1991 lifting equation, validation will require an extensive collaborative effort. Appropriate studies must be designed and conducted to determine whether the methods presented here effectively reduce the morbidity asso· ciated with manual materials handling, particularly two-handed lifting tasks.

9. Summary and conclusions The 1991 revised lifting equation was prepared as a methodological tool for safety and health practitioners who must evaluate the lifting demands of a wider range of manual handling jobs than contained in the 1981 Work Practices Guide for Manual Lifting (NIOSH 1981). The equation was designed to assist in the identification of ergonomic solutions for reducing the physical stresses associated with manuallifting by identify­ing the features of the lifting task that contribute the most to the hazard for low back injuries.

770 T. R. Waters et al.

Three criteria (biomecbanical, physiological, and psycbopbysical) were used to define tbe limiting components for tbe revised lifting equation. Tbis approacb was adopted because we found tbat a single criterion would likely fail 10 protect bealtby workers from back injury for many common types of lifting tasks. In general, tbe 1991 committee believed tbat tbe combination of using a multiplicative model and tbe practice of using the most conservative criterion or data values when faced with uncertainty served 10 provide a fina! lifting equation which is more Iikely 10 protect healtby workers for a wider variety of lifting tasks tban rnetbods which rely on only a single task factor (e.g., weight) Or single criterion (e.g., intradiscal pressure).

N10SH believes tbat tbe revised 1991 lifting equation is more Iikely tban tbe 1981 equation to protect most workers. There are two main reasons for this: (I) tbe 1991 equation is applicable to a wider variety of lifting jobs tban tbe 1981 equation because of tbe addition of tbe asymrnetric and coupling multipliers, ultimately affect­ing more lifting jobs and workers; and (2) tbe recommended weight Iimits computed using tbe 1991 equation are generally lower tban tbe maximum acceptable weight Iimits reported by Snook and Ciriello (1991). Because of tbe uncertainties in botb tbe existing scientific studies and tbeoretical models, furtber research is needed 10 assess tbe magnitude of risk for lifting-related LBP and its association witb tbe lifting index.

Acknowledgements The autbors gratefully acknowledge tbe technical assistance of tbe 1991 committee members in developing tbe revised equation: M. M. Ayoub, Ph.D.; Donald Chaffin, Ph.D.; Colin Drury, Ph. D.; Arun Garg, Ph.D.; and Suzanne Rodgers, Ph.D. We also wish 10 tbank Gunnar Andersson, M.D., Ph.D., Jerome Congleton, Ph.D., Stephan Konz, Ph.D., David Olson, Ph.D .. Stephen Simon, Ph.D., and Stover Snook, Ph.D., for tbeir valuable contributions in tbe review of tbe manuscript. Special tbanks to Anne C. Harnilton, Division of Standards Development and Technology Transfer, for ber care­fuI editorial review. Finally, special appreciation is extended to Janet C. Haartz, Ph.D., Director, Division of Biomedical and Behavioral Science, for her leadership, support, and technical guidance.

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Revised NIOSH equation 771

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muscle contraction forces: a double linear programming method, Journal of Biomech­anics. 21, 59--66.

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Received 2 September 1992. Revision accepted 4 November 1992.

Appendices A. Calcu1ation (or recommended welgbt Iimit RWL=LCxHMxVMxDMxAMxFMxCM

Recommended weight Iimil Component Metric Le = load constanl = 23 kg

HM = borizontal multiplier = (25/H) VM = vertical mu1tiplier = (I - (0·0031 V -751» DM = distance multiplier = (0·82 + (4·5/D» AM = asymmetric multiplier = (I - (0·OO32A» FM = frequency multiplier (from table 7) CM = coupling multiplier (from table 6)

where:

US customary 51 Ibs (IO/H)

(I - (0·00751 V - 301» (0·82 + (H/D) (1 - (0·OO32A»

H = horiwntal distance of hands from midpoinl hetween tbe ankles. Measure al tbe origin and tbe destination of tbe lifi (cm or in).

V = vertical distance of tbe hands from tbe lloor. Measure al tbe origin and destination of the lifi (cm or in).

D = vertical travel distance helween tbe origin and tbe destination of tbe Iifi (cm or in).

A = angle of asymmetry-angular displacernent of the load from tbe sagittal piane. Measure at tbe origin and destination of tbe lifi (degrees).

F = average frequency rate of lifting measured in liftslmin. Duration is defined lo be: S l h; S 2 h; or s; 8 h assuming appropriate recovery a110wances (see table 7).

Revised NIOSH equation 775

B. CaIculation for energy expenditure limi! l. For lifts above 75 cm (30 in), multiply the baseline aerobic work capacity (9·5 kcaV min)' by 0·7. 2. For lifting duration up to I h, multiply the value obtained in step I above by 0·5; for duration up to 2 h, multiply by 0·4; and, for duration between 2 and 8 h, multiply by 0·33.

For exarnple, the energy expenditure limit for 8 h of lifting above the waist (75 cm) would be 9·5 x 0·7 x 0·33 or 2·2 kcaVmin, as shown in table 3.

C. Comparison of criterion-based load weights Task descriptions Task I [floor-knuckle) Task 2 [knuckle-shoulder) Task 3 [shoulder-reach) Task 4 [floor-shoulder)

Common factors

H=42cm, V=Ocm, D=76cm, F= 1/30min H= 37 cm, V= 66 cm, D = 76cm, F= 1/30min H = 37 cm, V = 127 cm, D = 76cm, F = 4/min H=42cm, V= Ocm, D= 152cm, F=4/min

• 25th percentile female with a height of 160 cm and weight of 57 kg (Eastman Kodak 1986);

• semi-squat or stoop lifting posture; • box size of 40 x 34 x 14cm [LWH); • good couplings; • sagittal piane lifts only (no asymmetry); • lifting duration of 4 h.

To simplify the analyses. the following assumptions were made to correspond to the Snook and Ciriello (1991) data:

• vertical displacement (D) was assumed to be 76cm (30 inches); • box width (W) of 34 cm was chosen to correspond to Snooks' box width of

34 cm; • lifting duration of 4h was chosen to correspond to Snook and Ciriello (1991); • horizontal distance (H) was estimated from box width (W) and vertical lift

height (V) using the following equations:

H=20+ WI2 for V>75cm (30inches); H = 25 + W/2 for V < 75 cm (30 inches).

Basis far determining criterion-based weight limits The University of Michigan 2D SSPP Prograrn was used to determine biomechani­cally-based load weights that produce a disc compression of 350 kgs (3·4 kN) (i.e., the biomechanical criterioo).

The University of Michigan Energy Expenditure Prediction Program was used to determine the physiologically-based load weights that produce energy expenditures equivalent to those displayed in table 3 for a lifting duration of 2-8 h. For exarnple, where V is below 75 cm (tasks l, 2, and 4), 3·1 kcaVmin was used, where V is above 75 cm (task 3), 2·2 kcaVmin was used.

6 The 9·5 kcallmin baseline aerobic capacity value is equivalent lo 90% of a 10·5 kcaVmin baseline aerobic capacity for treadrnill activity.

776 Revised NIOSH equation

The psychophysically-based load weights for Tasks 1-3 were taken from Snook and Ciriello's (1991) female lifting database. The load weights are equivalent to the values that are acceptable to 75% of the female population for a 34cm box width, 76 cm vertical displacement, and a lifting frequency of 4 Iifis/min. For task 4, the load weight is taken from Ayoub et al. (1978) (table 8, p. 77, adjusted for 75% female acceptable ).

D. Equations nsed lo estimate energy expenditure from Garg (1976) The following equations from Garg (1976) were used to estimate energy expenditure:

Stoop lift E=O·OI09 BW+ (0·0012 BW+0·0052 L+0·0028 SxL)f (I)

Squat Iift E=O·OI09 BW+ (0·0019 BW+0·0081 L+0·0023 Sx L)f (2)

Arm Iift E= 0·0109 BW + (0-0002 BW + 0·0103 L- ().0017 Sx L)f (3)

where:

E = energy expenditure (kcaI/min) BW = body weight Obs)

L = weight of the load (Ibs) S = sex (female = O, male = l) f = frequency of lifting (lifis/min)

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GRAND TOTAL ,~ Over $100.00 ..................... $8.00

I I

Ali previous versions of this arder form Md $2.00 IO aboYe !or orders seni Prices are 8ubject lo change. ... _. CU5Ida ofthe u.S .• Canada. and MEDOCO The NTlS SaIes Oesk (700) ~7-4660